bulletproof some NS_RELEASE's with NS_IF_RELEASE.

netwerk/base/src/nsAsyncStreamListener.cpp

@@ -128,15 +128,15 @@ nsStreamListenerEvent::nsStreamListenerEvent(nsAsyncStreamObserver* listener,
                                              nsIChannel* channel, nsISupports* context)
     : mListener(listener), mChannel(channel), mContext(context)
 {
-    NS_ADDREF(mListener);
+    NS_IF_ADDREF(mListener);
-    NS_ADDREF(mChannel);
+    NS_IF_ADDREF(mChannel);
     NS_IF_ADDREF(mContext);
 }
 
 nsStreamListenerEvent::~nsStreamListenerEvent()
 {
-    NS_RELEASE(mListener);
+    NS_IF_RELEASE(mListener);
-    NS_RELEASE(mChannel);
+    NS_IF_RELEASE(mChannel);
     NS_IF_RELEASE(mContext);
 }