If the fmt being parsed by block_size is exactly '%', *++fmt is the
terminator and strchr will return a valid pointer to the terminator of
spec rather than NULL, the while condition will pass and subsequent
strchr will read past the end of fmt until a spec character is found
again

ASAN aborts with the following error on the first buffer overread:

AddressSanitizer: heap-buffer-overflow on address 0x602000000212 at pc 0x55bf1c4b2d78 bp 0x7ffe33c8cff0 sp 0x7ffe33c8cfe0
READ of size 1 at 0x602000000212 thread T0
    #0 0x55bf1c4b2d77 in block_size ../text-utils/hexdump-parse.c:207
    #1 0x55bf1c4ad36f in main ../text-utils/hexdump.c:214
    #2 0x7f15f063c78f  (/usr/lib/libc.so.6+0x2378f)
    #3 0x7f15f063c849 in __libc_start_main (/usr/lib/libc.so.6+0x23849)
    #4 0x55bf1c4ac6c4 in _start (../build/hexdump+0x86c4)