libmedia: check NULL pointer at binder receiver side.

Change-Id: I464da96f3d6081dd8c3cc2fd977e2b96f5d477bb
diff --git a/media/libmediaplayerservice/MediaPlayerService.cpp b/media/libmediaplayerservice/MediaPlayerService.cpp
index a416381..dda0a57 100644
--- a/media/libmediaplayerservice/MediaPlayerService.cpp
+++ b/media/libmediaplayerservice/MediaPlayerService.cpp
@@ -1246,7 +1246,10 @@
             if (client->mAudioOutput != NULL)
                 client->mAudioOutput->switchToNextOutput();
             client->mNextClient->start();
-            client->mNextClient->mClient->notify(MEDIA_INFO, MEDIA_INFO_STARTED_AS_NEXT, 0, obj);
+            if (client->mNextClient->mClient != NULL) {
+                client->mNextClient->mClient->notify(
+                        MEDIA_INFO, MEDIA_INFO_STARTED_AS_NEXT, 0, obj);
+            }
         }
     }