Null-check argument to ConnectivityManager.releaseNetworkRequest(PendingIntent)

Do this in ConnectivityService to prevent crashing framework.

Bug:21329396
Change-Id: Ifd9c970cd0da392d0bd5da9a9ae845fdf0b559b3
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index 7ac2655..edca4b8 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -3640,6 +3640,7 @@
 
     @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));
     }