Bug 1425866: Just null-check the CSS Loader for now. r=heycam

After discussing with Olli there isn't any kind of severe problem out of this.
Shadow subtrees will be disconnected just like the rest, and they shouldn't
assume that the document hasn't been disconnected first.

MozReview-Commit-ID: CX4fXOqEIFj

--HG--
extra : rebase_source : cd30cb8b8199fb73120c0bcade68986454090005

dom/base/nsStyleLinkElement.cpp

@@ -256,7 +256,8 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
 
   nsCOMPtr<nsIDocument> doc = thisContent->IsInShadowTree() ?
     thisContent->OwnerDoc() : thisContent->GetUncomposedDoc();
-  if (!doc || !doc->CSSLoader()->GetEnabled()) {
+  // Loader could be null during unlink, see bug 1425866.
+  if (!doc || !doc->CSSLoader() || !doc->CSSLoader()->GetEnabled()) {
     return Update { };
   }