song_save: don't call tag_free(NULL)

When a song was in the database twice (which shouldn't happen), and
the first song had no tag items, MPD calledd tag_free(NULL).  Add a
check to that source location, and an assertion to tag_free().

src/song_save.c

@@ -80,7 +80,8 @@ insertSongIntoList(struct songvec *sv, struct song *newsong)
 			tag_end_add(newsong->tag);
 	} else { /* prevent dupes, just update the existing song info */
 		if (existing->mtime != newsong->mtime) {
-			tag_free(existing->tag);
+			if (existing->tag != NULL)
+				tag_free(existing->tag);
 			if (newsong->tag)
 				tag_end_add(newsong->tag);
 			existing->tag = newsong->tag;

src/tag.c

@@ -163,6 +163,8 @@ void tag_free(struct tag *tag)
 {
 	int i;
 
+	assert(tag != NULL);
+
 	g_mutex_lock(tag_pool_lock);
 	for (i = tag->num_items; --i >= 0; )
 		tag_pool_put_item(tag->items[i]);