style: Don't let @namespace rules that aren't going to be inserted affect the namespace map.

Bug: 1464865
Reviewed-by: xidorn
MozReview-Commit-ID: 9bjlEBExqsr

components/style/stylesheets/stylesheet.rs

@@ -368,7 +368,8 @@ impl Stylesheet {
             context,
             error_context,
             state: State::Start,
-            had_hierarchy_error: false,
+            dom_error: None,
+            insert_rule_context: None,
             namespaces,
         };