output/{alsa,oss}: add assertions

src/output/AlsaOutputPlugin.cxx

@@ -802,6 +802,7 @@ alsa_play(struct audio_output *ao, const void *chunk, size_t size,
 {
 	AlsaOutput *ad = (AlsaOutput *)ao;
 
+	assert(size > 0);
 	assert(size % ad->in_frame_size == 0);
 
 	if (ad->must_prepare) {
@@ -819,6 +820,7 @@ alsa_play(struct audio_output *ao, const void *chunk, size_t size,
 	assert(size % ad->out_frame_size == 0);
 
 	size /= ad->out_frame_size;
+	assert(size > 0);
 
 	while (true) {
 		snd_pcm_sframes_t ret = ad->writei(ad->pcm, chunk, size);

src/output/OssOutputPlugin.cxx

@@ -727,6 +727,8 @@ oss_output_play(struct audio_output *ao, const void *chunk, size_t size,
 	OssOutput *od = (OssOutput *)ao;
 	ssize_t ret;
 
+	assert(size > 0);
+
 	/* reopen the device since it was closed by dropBufferedAudio */
 	if (od->fd < 0 && !oss_reopen(od, error))
 		return 0;
@@ -735,6 +737,8 @@ oss_output_play(struct audio_output *ao, const void *chunk, size_t size,
 	chunk = od->pcm_export->Export(chunk, size, size);
 #endif
 
+	assert(size > 0);
+
 	while (true) {
 		ret = write(od->fd, chunk, size);
 		if (ret > 0) {