music_pipe: refuse to push empty chunks

src/pipe.c

@@ -76,6 +76,8 @@ music_pipe_shift(struct music_pipe *mp)
 
 	chunk = mp->head;
 	if (chunk != NULL) {
+		assert(!music_chunk_is_empty(chunk));
+
 		mp->head = chunk->next;
 		--mp->size;
 
@@ -112,6 +114,8 @@ music_pipe_clear(struct music_pipe *mp, struct music_buffer *buffer)
 void
 music_pipe_push(struct music_pipe *mp, struct music_chunk *chunk)
 {
+	assert(!music_chunk_is_empty(chunk));
+
 	g_mutex_lock(mp->mutex);
 
 	chunk->next = NULL;