In spdif mode, there are hacks that try to cut audio on frame boundaries
(blame spdif, which is a hack in itself). The "alignment" is used in a
bunch of places, but --end does not respect it. This leads to some audio
that can't be pushed because the alignment is off (I don't know why, not
do I care), which puts audio into an underrun state forever.
Fix this by discarding unusable extra samples if no new data can be
expected.
Fixes: #6935