LibWeb: Make update_style() a no-op if there's no browsing context

Style is needed to lay out and paint a document, but we can't do either
those when the document isn't attached to a browsing context.

Author: awesomekling

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -445,6 +445,8 @@ static void update_style_recursively(DOM::Node& node)
 
 void Document::update_style()
 {
+    if (!browsing_context())
+        return;
     update_style_recursively(*this);
     update_layout();
 }