Fix null dereferences in the dis and pyc disassemblers without a loaded file ##crash - #27139

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Description

two disassemblers crash when there is no file behind the bytes , as with rasm2 -d :

$ rasm2 -a dis -b 32 -d 1e2e67cb122a
ERROR: AddressSanitizer: SEGV on unknown address 0x000000000050
    #0 in decode libr/arch/p/dis/plugin.c:305
$ rasm2 -a pyc -d 8eaf2062
ERROR: AddressSanitizer: SEGV on unknown address 0x000000000008
    #0 in pyc_decode libr/arch/p/pyc/plugin.c:170

• dis only sets o when the dis bin plugin is loaded , but jmp , call and the conditional branches read o->pcs anyway . a small resolve_pc returns UT64_MAX when there is no module , which is what a pc that is not in the table already gave
• pyc read b->cur with no RBin , that is R_UNWRAP4 now . after that it passed the unset asm.cpu to strcmp in get_opcode_by_version , so with no cpu it takes the same v3.9.0 table an unknown version already falls back to . without the pyc file there are no code objects , so the bytes come out as invalid , as they do inside r2 on a raw buffer

found with an ASan build , disassembling random 1 to 16 byte strings with every plugin rasm2 -L lists that can disassemble , 100 per plugin . these two were the only reports , with the change the same 8500 runs report nothing . the 4 lines in db/asm/dis_32 and db/asm/pyc crash before the change .