Zero out the return value from QI in case of failure.

view/src/nsScrollingView.cpp

@@ -395,7 +395,7 @@ nsresult nsScrollingView::QueryInterface(const nsIID& aIID, void** aInstancePtr)
   if (nsnull == aInstancePtr) {
     return NS_ERROR_NULL_POINTER;
   }
-  
+  *aInstancePtr = nsnull;
   if (aIID.Equals(kIScrollableViewIID)) {
     *aInstancePtr = (void*)(nsIScrollableView*)this;
     return NS_OK;

view/src/nsView.cpp

@@ -151,6 +151,9 @@ nsresult nsView :: QueryInterface(const nsIID& aIID, void** aInstancePtr)
   if (nsnull == aInstancePtr) {
     return NS_ERROR_NULL_POINTER;
   }
+  
+  *aInstancePtr = nsnull;
+  
   static NS_DEFINE_IID(kISupportsIID, NS_ISUPPORTS_IID);
   static NS_DEFINE_IID(kClassIID, kIViewIID);