Null check for CSSGlobalRuleSet.

Presumably fixes UBSAN crasher. Seemingly so running locally with fuzzer
test case.

Bug: 904037
Change-Id: Ie5804e28835528e65caadbf43308260b961a92b7
Reviewed-on: https://chromium-review.googlesource.com/c/1358446Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613155}

third_party/blink/renderer/core/css/style_engine.cc

@@ -529,7 +529,8 @@ void StyleEngine::ResetAuthorStyle(TreeScope& tree_scope) {
   if (!scoped_resolver)
     return;
 
-  global_rule_set_->MarkDirty();
+  if (global_rule_set_)
+    global_rule_set_->MarkDirty();
   if (tree_scope.RootNode().IsDocumentNode()) {
     scoped_resolver->ResetAuthorStyle();
     return;