fix uninitialized variable

xpcom/ds/nsAtomService.cpp

@@ -26,7 +26,9 @@
 NS_IMPL_THREADSAFE_ISUPPORTS1(nsAtomService, nsIAtomService)
 
 nsAtomService::nsAtomService()
-{}
+{
+  NS_INIT_ISUPPORTS();
+}
 
 nsAtomService::~nsAtomService()
 {}