[Fix] Possible out-of-bounds read when computing length of some IT files with pattern loops (OpenMPT: formats that are converted to IT, libopenmpt: IT/ITP/MO3), caught with afl-fuzz.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@10027 56274372-70c3-4bfc-bfc3-4c3a0b034d27