check for ISupports in QI

mailnews/base/util/nsMsgMailNewsUrl.cpp

@@ -90,6 +90,12 @@ nsresult nsMsgMailNewsUrl::QueryInterface(const nsIID &aIID, void** aInstancePtr
 		return NS_OK;
 	}
  
+  if (aIID.Equals(nsCOMTypeInfo<nsISupports>::GetIID()))    
+	{
+		*aInstancePtr = (void *) ((nsIMsgMailNewsUrl*) this);
+		AddRef();
+		return NS_OK;
+	}
 #if defined(NS_DEBUG)
     /*
      * Check for the debug-only interface indicating thread-safety