There was a discrepancy between what was written to the buffer and the
size returned by pcm_dsd_to_dop(): the "for" loop uses num_frames/2,
rounding down, while the return value is num_samples which is
num_frames*channels, without rounding.  This could cause undefined
data at the end of the destination buffer if the source buffer size
was not aligned to multiples of 8 bytes (4 DSD bytes per channel).

The latter however can occur in the 0.21 branch after commit
a06bf388

Closes #233