load_abc.cpp (ReadABC): fix -Waddress warning from gcc14

[sezero]

load_abc.cpp: In member function 'BOOL CSoundFile::ReadABC(const uint8_t*, DWORD)':
load_abc.cpp:3958:55: warning: comparing the result of pointer addition '(pm + 2)' and NULL [-Waddress]
 3958 |                                                 if (pm+2)
      |                                                     ~~^~