audio: make playback end with --end and --audio-spdif

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

player/audio.c

@@ -975,6 +975,9 @@ void fill_audio_out_buffers(struct MPContext *mpctx)
 
     audio_eof &= partial_fill;
 
+    if (audio_eof && playsize < align)
+        playsize = 0;
+
     // With gapless audio, delay this to ao_uninit. There must be only
     // 1 final chunk, and that is handled when calling ao_uninit().
     if (audio_eof && !opts->gapless_audio)