output_buffer: don't split frames

Splitting a frame between two buffer chunks causes distortion in the
output.  MPD used to assume that the chunk size 1020 would never cause
splitted frames, but that isn't the case for 24 bit stereo (127.5
frames), and even less for files with even more channels.

src/pcm_utils.c

@@ -560,6 +560,9 @@ size_t pcm_convert_size(const struct audio_format *inFormat, size_t src_size,
 	const int shift = 2 * outFormat->channels;
 	size_t dest_size = src_size;
 
+	/* no partial frames allowed */
+	assert((src_size % audio_format_frame_size(inFormat)) == 0);
+
 	switch (inFormat->bits) {
 	case 8:
 		dest_size <<= 1;