fix for 83363 - crash if insertNode tries to insert null node

r=mjudge@netscape.com
sr=kin@netscape.com
a=asa@mozilla.org

content/base/src/nsRange.cpp

@@ -1743,8 +1743,9 @@ nsresult nsRange::InsertNode(nsIDOMNode* aN)
   if(IsDetached())
     return NS_ERROR_DOM_INVALID_STATE_ERR;
 
-  nsresult res;
+  NS_ENSURE_ARG_POINTER(aN);
  
+  nsresult res;
   PRInt32 tStartOffset;
   this->GetStartOffset(&tStartOffset);