media library: fix sorting by disc

src/media_library.cpp

@@ -940,7 +940,7 @@ void DisplayPrimaryTags(NC::Menu<std::string> &menu)
 
 bool SortSongsByTrack(const MPD::Song &a, const MPD::Song &b)
 {
-	int cmp = a.getDisc().compare(a.getDisc());
+	int cmp = a.getDisc().compare(b.getDisc());
 	if (cmp != 0)
 		return cmp;
 	return a.getTrack() < b.getTrack();