clear the category's variable tail pointer as well when variables are detached from it

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@47686 65c4cc65-6c06-0410-ace0-fbb531ad65f3

config.c

@@ -301,6 +301,7 @@ struct ast_variable *ast_category_detach_variables(struct ast_category *cat)
 
 	v = cat->root;
 	cat->root = NULL;
+	cat->last = NULL;
 
 	return v;
 }