ReadMOD(): add name string sanity checks to 15 instrument MOD loader so that it doesn't accept any binary data.

ReadMOD(): add name string sanity checks to 15 instrument MOD loader
so that it doesn't accept any binary data.

This replaces the old pull request #8 which used to kill 15 instrument mod loading.
libmikmod also ckecks finetune values, but I didn't do that here. ReadMOD now
seems to behave when fed with random binary data.