Previously AudioPlaylist::read always returned the timecnt that
it was supposed to read into the buffer, regardless if the given
number of samples was read. The check in DiskReader::refill_audio
`if (nread != to_read)` never triggered.
This can happen when changing an audio-region's time-domain to
music-time (glue to bars/beats). Region-length (beats converted
to samples) can exceed the actual audio-source length (in samples).
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