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/audio.c

@@ -254,6 +254,9 @@ int playAudio(const char *buffer, size_t length)
 	int ret = -1, err;
 	unsigned int i;
 
+	/* no partial frames allowed */
+	assert((length % audio_format_frame_size(&input_audio_format)) == 0);
+
 	syncAudioDeviceStates();
 
 	for (i = 0; i < audioOutputArraySize; ++i)