Null-check argument to ConnectivityManager.releaseNetworkRequest(PendingIntent)

Do this in ConnectivityService to prevent crashing framework.

Bug:21329396
Change-Id: Ifd9c970cd0da392d0bd5da9a9ae845fdf0b559b3

services/core/java/com/android/server/ConnectivityService.java

@@ -3640,6 +3640,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
 
     @Override
     public void releasePendingNetworkRequest(PendingIntent operation) {
+        checkNotNull(operation, "PendingIntent cannot be null.");
         mHandler.sendMessage(mHandler.obtainMessage(EVENT_RELEASE_NETWORK_REQUEST_WITH_INTENT,
                 getCallingUid(), 0, operation));
     }