Edge case / off by 1 error in bdshemu

[ScimitarEnjoyer]

When an instruction cannot be emulated, the emulator sets stop to true, and on the next iteration returns SHEMU_ABORT_CANT_EMULATE.

while (Context->InstructionsCount++ < Context->MaxInstructionsCount)
{
    SHEMU_STATUS shstatus;
    NDSTATUS ndstatus;
    ND_UINT64 rip;          // Offset, not actual linear address.
    ND_UINT32 i, ilen;
    ND_UINT8 ibytes[16];
    ND_BOOL hasNd = ND_FALSE;

    tsc++;

    // Reset all the operands to 0.
    nd_memzero(&res, sizeof(res));
    nd_memzero(&dst, sizeof(dst));
    nd_memzero(&src, sizeof(src));

    // The stop flag has been set, this means we've reached a valid instruction, but that instruction cannot be
    // emulated (for example, SYSCALL, INT, system instructions, etc).
    if (stop)
    {
        return SHEMU_ABORT_CANT_EMULATE;
    }

In cases where the last instruction cannot be emulated, this causes the emulator to return SHEMU_SUCCESS instead of the expected SHEMU_ABORT_CANT_EMULATE.

How to reproduce this issue
Set up the shellcode to emulate only 1 instruction (MaxInstructionsCount = 1) and point it to an instruction it doesn't support (like a syscall). The emulator will return SHEMU_SUCCESS instead of SHEMU_ABORT_CANT_EMULATE because the max instruction count check precedes the previous instruction unsupported check.

I have created a pull request for this issue:
#107