Fix crash after detaching document.

Source/core/html/HTMLLabelElement.cpp

Index: Source/core/html/HTMLLabelElement.cpp
diff --git a/Source/core/html/HTMLLabelElement.cpp b/Source/core/html/HTMLLabelElement.cpp
index b625e78f6d1c03e85537220ce833a8da5c2d4f3e..13ca30e6b67ee2cdf7624d7be1bfc9d5a188adaa 100644
--- a/Source/core/html/HTMLLabelElement.cpp
+++ b/Source/core/html/HTMLLabelElement.cpp
@@ -140,8 +140,10 @@ void HTMLLabelElement::defaultEventHandler(Event* evt)
     if (evt->type() == EventTypeNames::click && !processingClick) {
         // If text of label element is selected, do not pass
         // the event to control element.
-        if (document().frame()->selection().selection().isRange())
-            return;
+        if (LocalFrame* frame = document().frame()) {
+            if (frame->selection().selection().isRange())
+                return;
+        }
 
         RefPtrWillBeRawPtr<HTMLElement> element = control();