libmodplug: Integer overflow when reading samples of AMF files

Multiple buffer overflows in MODPlug Tracker (OpenMPT) 1.17.02.43 and earlier and libmodplug 0.8 and earlier, as used in GStreamer and possibly other products, allow user-assisted remote attackers to execute arbitrary code via (1) long strings in ITP files used by the CSoundFile::ReadITProject function in soundlib/Load_it.cpp and (2) crafted modules used by the CSoundFile::ReadSample function in soundlib/Sndfile.cpp, as demonstrated by crafted AMF files.