MediaPlayer: Sanitize supplier/mediaFile management

* Fix a leak.

src/apps/mediaplayer/playlist/PlaylistItem.cpp

@@ -199,6 +199,7 @@ bigtime_t PlaylistItem::_CalculateDuration()
 
 	delete vi;
 	delete au;
+	ReleaseTrackSupplier();
 
 	return duration;
 }

src/apps/mediaplayer/playlist/UrlPlaylistItem.cpp

@@ -139,12 +139,11 @@ UrlPlaylistItem::_CreateTrackSupplier() const
 		return NULL;
 
 	BMediaFile* mediaFile = new(std::nothrow) BMediaFile(fUrl);
-	if (mediaFile == NULL) {
+	if (mediaFile == NULL || supplier->AddMediaFile(mediaFile) != B_OK) {
+		delete mediaFile;
 		delete supplier;
 		return NULL;
 	}
-	if (supplier->AddMediaFile(mediaFile) != B_OK)
-			delete mediaFile;
 
 	return supplier;
 }