Defensive programming for UnregisterObserver after Shutdown. r=dbaron

xpcom/base/nsMemoryImpl.cpp

@@ -159,7 +159,9 @@ nsMemoryImpl::RegisterObserver(nsIMemoryPressureObserver* obs)
 NS_IMETHODIMP
 nsMemoryImpl::UnregisterObserver(nsIMemoryPressureObserver* obs)
 {
-    NS_ASSERTION(mObservers, "never called RegisterObserver");
+    NS_ASSERTION(mObservers, "never called nsIMemory::RegisterObserver or called after nsIMemory::Shutdown");
+    if (!mObservers)
+        return NS_OK;
     return mObservers->RemoveElement(obs);
 }