opl note limitation and octave overflow fixes
Update i_oplmusic.c
Merge pull request #466 from khokh2001/opl-fix2

opl note limitation and octave overflow fixes

Adjust how the OPL MIDI code behaves at extreme MIDI note values
(high/low octaves) to better match how the Doom DMX library decides
on the OPL register value (thanks Alexey Khokholov / khokh20010.