db/simple/Song: make "parent" a reference, not a pointer

src/db/update/Editor.cxx

@@ -29,7 +29,7 @@
 void
 DatabaseEditor::DeleteSong(Directory &dir, Song *del)
 {
-	assert(del->parent == &dir);
+	assert(&del->parent == &dir);
 
 	/* first, prevent traversers in main task from getting this */
 	dir.RemoveSong(del);
@@ -65,7 +65,7 @@ DatabaseEditor::ClearDirectory(Directory &directory)
 		});
 
 	directory.ForEachSongSafe([this, &directory](Song &song){
-			assert(song.parent == &directory);
+			assert(&song.parent == &directory);
 			DeleteSong(directory, &song);
 		});
 }

src/db/update/Walk.cxx

@@ -79,7 +79,7 @@ UpdateWalk::RemoveExcludedFromDirectory(Directory &directory,
 		});
 
 	directory.ForEachSongSafe([&](Song &song){
-			assert(song.parent == &directory);
+			assert(&song.parent == &directory);
 
 			const auto name_fs = AllocatedPath::FromUTF8(song.uri.c_str());
 			if (name_fs.IsNull() || exclude_list.Check(name_fs)) {