f6a4a52 Focus the lock check (ja)
d9d1652 Add critcal section to UpdateStatusInternal() (blondfrogs)
48c58da Fix formatting of ztracker remove pending (blondfrogs)

Pull request description:

  The following segfault occurred on a node running veil. This is a potential fix to this segfault

  ```
  [Thread 0x7ffef37fe700 (LWP 36705) exited]
  [New Thread 0x7ffef37fe700 (LWP 36914)]
  [Thread 0x7ffef37fe700 (LWP 36914) exited]
  [New Thread 0x7ffef37fe700 (LWP 37562)]
  [Thread 0x7ffef37fe700 (LWP 37562) exited]
  [New Thread 0x7ffef37fe700 (LWP 38217)]
  [Thread 0x7ffef37fe700 (LWP 38217) exited]
  double free or corruption (fasttop)

  Thread 26 "veil-httpworker" received signal SIGABRT, Aborted.
  [Switching to Thread 0x7fff98ff9700 (LWP 45833)]
  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
  51        ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
  (gdb) backtrace
  #0  0x00007ffff2f22e97 in __GI_raise (sig=sig@entry=6)
      at ../sysdeps/unix/sysv/linux/raise.c:51
  #1  0x00007ffff2f24801 in __GI_abort () at abort.c:79
  #2  0x00007ffff2f6d897 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7ffff309ab9a "%s\n") at ../sysdeps/posix/libc_fatal.c:181
  #3  0x00007ffff2f7490a in malloc_printerr (str=str@entry=0x7ffff309c828 "double free or corruption (fasttop)") at malloc.c:5350
  #4  0x00007ffff2f7c004 in _int_free (have_lock=0, p=0x7ffef8155530, av=0x7ffef8000020) at malloc.c:4230
  #5  0x00007ffff2f7c004 in __GI___libc_free (mem=0x7ffef8155540)
      at malloc.c:3124
  #6  0x000055555597b93b in __gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<uint256 const, uint256> > >::deallocate(std::_Rb_tree_node<std::pair<uint256 const, uint256> >*, unsigned long) (this=<optimized out>, __p=0x7ffef8155540)
      at /usr/include/c++/7/ext/new_allocator.h:125
  #7  0x000055555597b93b in std::allocator_traits<std::allocator<std::_Rb_tree_node<std::pair<uint256 const, uint256> > > >::deallocate(std::allocator<std::_Rb_tree_node<std::pair<uint256 const, uint256> > >&, std::_Rb_tree_node<std::pair<uint256 const, uint256> >*, unsigned long) (__a=..., __n=1, __p=0x7ffef8155540)
      at /usr/include/c++/7/bits/alloc_traits.h:462

  #8  0x000055555597b93b in std::_Rb_tree<uint256, std::pair<uint256 const, uint256>, std::_Select1st<std::pair<uint256 const, uint256> >, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::_M_put_node(std::_Rb_tree_nod---Type <return> to continue, or q <return> to quit---
  e<std::pair<uint256 const, uint256> >*) (this=0x555556c0c728, __p=0x7ffef8155540) at /usr/include/c++/7/bits/stl_tree.h:592
  #9  0x000055555597b93b in std::_Rb_tree<uint256, std::pair<uint256 const, uint256>, std::_Select1st<std::pair<uint256 const, uint256> >, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::_M_drop_node(std::_Rb_tree_node<std::pair<uint256 const, uint256> >*) (this=0x555556c0c728, __p=0x7ffef8155540) at /usr/include/c++/7/bits/stl_tree.h:659
  #10 0x000055555597b93b in std::_Rb_tree<uint256, std::pair<uint256 const, uint256>, std::_Select1st<std::pair<uint256 const, uint256> >, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::_M_erase(std::_Rb_tree_node<std::pair<uint256 const, uint256> >*) (this=this@entry=0x555556c0c728, __x=0x7ffef8155540) at /usr/include/c++/7/bits/stl_tree.h:1858
  #11 0x0000555555baeef5 in std::_Rb_tree<uint256, std::pair<uint256 const, uint256>, std::_Select1st<std::pair<uint256 const, uint256> >, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::clear() (this=0x555556c0c728) at /usr/include/c++/7/bits/stl_tree.h:1171

  #12 0x0000555555baeef5 in std::_Rb_tree<uint256, std::pair<uint256 const, uint256>, std::_Select1st<std::pair<uint256 const, uint256> >, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::_M_erase_aux(std::_Rb_tree_const_iterator<std::pair<uint256 const, uint256> >, std::_Rb_tree_const_iterator<std::pair<uint256 const, uint256> >) (__last=
        {first = {<base_blob<256>> = {static WIDTH = 32, data = '\000' <repeats 25 times>, "\247\356\071\377\177\000"}, <No data fields>}, second = {<base_blob<25---Type <return> to continue, or q <return> to quit---
  6>> = {static WIDTH = 32, data = "p\b\337\071\377\177\000\000\000\365\372\071\377\177\000\000\267\017", '\000' <repeats 13 times>}, <No data fields>}}, __first={first = {<base_blob<256>> = {static WIDTH = 32, data = "\005\371\254|\260/\220\353ed# \221\221\025\210\365_Y\216\367i=\352\231\006\016\202`ٜ"}, <No data fields>}, second = {<base_blob<256>> = {static WIDTH = 32, data = "ŏ9c\204\210\274\060C{\337k\357bۉ\033\026\347\001z\030\200\240\256!\275\251\316\002", <incomplete sequence \347>}, <No data fields>}}, this=0x555556c0c728)
      at /usr/include/c++/7/bits/stl_tree.h:2488
  #13 0x0000555555baeef5 in std::_Rb_tree<uint256, std::pair<uint256 const, uint256>, std::_Select1st<std::pair<uint256 const, uint256> >, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::erase(uint256 const&) (this=this@entry=0x555556c0c728, __x=...) at /usr/include/c++/7/bits/stl_tree.h:2502
  #14 0x0000555555ba86dd in std::map<uint256, uint256, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > >::erase(uint256 const&) (__x=..., this=0x555556c0c728) at /usr/include/c++/7/bits/stl_map.h:1062
  #15 0x0000555555ba86dd in CzTracker::RemovePending(uint256 const&) (this=this@entry=0x555556c0c6e0, txid=...) at veil/zerocoin/ztracker.cpp:360
  #16 0x0000555555ba933f in CzTracker::UpdateStatusInternal(std::set<uint256, std::less<uint256>, std::allocator<uint256> > const&, std::map<uint256, uint256, std::less<uint256>, std::allocator<std::pair<uint256 const, uint256> > > const&, CMintMeta&) (this=this@entry=0x555556c0c6e0, setMempool=std::set with 2 elements = {...}, mapMempoolSerials=std::map with 0 elements, mint=...)
      at veil/zerocoin/ztracker.cpp:386
  ---Type <return> to continue, or q <return> to quit---
  #17 0x0000555555bacaf6 in CzTracker::ListMints(bool, bool, bool) (this=0x555556c0c6e0, fUnusedOnly=fUnusedOnly@entry=false, fMatureOnly=fMatureOnly@entry=false, fUpdateStatus=fUpdateStatus@entry=true) at veil/zerocoin/ztracker.cpp:503
  #18 0x0000555555b132ee in CWallet::ListMints(bool, bool, bool) (this=this@entry=0x7fff3e6b18f0, fUnusedOnly=fUnusedOnly@entry=false, fMatureOnly=fMatureOnly@entry=false, fUpdateStatus=fUpdateStatus@entry=true) at wallet/wallet.cpp:2661
  #19 0x0000555555b02b44 in ResetMints(CWallet*) (pwallet=pwallet@entry=0x7fff3e6b18f0) at wallet/rpczerocoin.cpp:789
  #20 0x0000555555b05afa in rescanzerocoinwallet(JSONRPCRequest const&) (request=...) at wallet/rpczerocoin.cpp:905
  #21 0x000055555592de67 in CRPCTable::execute(JSONRPCRequest const&) const (this=<optimized out>, request=...) at rpc/server.cpp:496
  #22 0x0000555555a381ef in HTTPReq_JSONRPC(HTTPRequest*, std::__cxx11::string const&) (req=0x7ffee4766b50) at httprpc.cpp:194
  #23 0x0000555555a3abdc in std::function<bool (HTTPRequest*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>::operator()(HTTPRequest*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const (__args#1="", __args#0=<optimized out>, this=0x7ffee59abaa0) at /usr/include/c++/7/bits/std_function.h:706
  #24 0x0000555555a3abdc in HTTPWorkItem::operator()() (this=0x7ffee59aba70)
      at httpserver.cpp:54
  #25 0x0000555555a3abdc in WorkQueue<HTTPClosure>::Run() (this=0x7fffbc001b00)
      at httpserver.cpp:113
  ---Type <return> to continue, or q <return> to quit---
  #26 0x0000555555a3abdc in HTTPWorkQueueRun(WorkQueue<HTTPClosure>*) (queue=0x7fffbc001b00) at httpserver.cpp:334
  #27 0x00007ffff394866f in  () at /usr/lib/x86_64-linux-gnu/libstdc++.so.6
  #28 0x00007ffff4d646db in start_thread (arg=0x7fff98ff9700)
      at pthread_create.c:463
  #29 0x00007ffff300588f in clone ()
      at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
  (gdb)

  ```

Tree-SHA512: cc9aa446f7c30a8cd948a6c93cd93ec22f757cfd0a0cfe7a6eadfdf0013619af57a37d25de746c0b3cf1bb8d5656d91e3ceea7b7bb943643dacd0fa64d724ea3