output: don't allow length==0

Nobody should call playAudio() with an empty chunk.  Add some
assertions on that.

src/audio.c

@@ -228,6 +228,7 @@ bool playAudio(const char *buffer, size_t length)
 	bool ret = false;
 	unsigned int i;
 
+	assert(length > 0);
 	/* no partial frames allowed */
 	assert((length % audio_format_frame_size(&input_audio_format)) == 0);
 

src/output_control.c

@@ -107,6 +107,8 @@ audio_output_signal(struct audio_output *ao)
 void audio_output_play(struct audio_output *audioOutput,
 		       const char *playChunk, size_t size)
 {
+	assert(size > 0);
+
 	if (!audioOutput->open)
 		return;
 

src/output_thread.c

@@ -65,6 +65,8 @@ static void ao_play(struct audio_output *ao)
 	size_t size = ao->args.play.size;
 	bool ret;
 
+	assert(size > 0);
+
 	if (!audio_format_equals(&ao->inAudioFormat, &ao->outAudioFormat))
 		convertAudioFormat(ao, &data, &size);