AUDIO: Do not error out when channel offset >= length after interrupt()

This fixes a Protracker module in the OS/2 version of
Hopkins FBI (bug #3612101). In row 0x30 of the first
pattern, the set channel offset effect in the fourth track
pushes the offset past the sample (repeat) length.
This is not error; the mixing function already handles this
case flawlessly. No assert() is necessary there.