don't assign a nsCOMPtr to null. use null_nsCOMPtr()

mailnews/addrbook/src/nsAbRDFResource.cpp

@@ -35,7 +35,7 @@ nsAbRDFResource::nsAbRDFResource(void)
 {
 	NS_INIT_REFCNT();
 
-	mDatabase = nsnull;
+	mDatabase = null_nsCOMPtr();
 	GetAbDatabase();
 }