Add missing return statement

Test: Builds successfully
Change-Id: Ic785fe69dd085f5a1920a068672073c2e21e00a2

media/java/android/media/MediaRouter2.java

@@ -310,6 +310,7 @@ public class MediaRouter2 {
             List<MediaRoute2Info> outRoutes) {
         if (provider == null || !provider.isValid()) {
             Log.w(TAG, "Ignoring invalid provider : " + provider);
+            return;
         }
 
         final Collection<MediaRoute2Info> routes = provider.getRoutes();