Don't crash when importing a 404

sky/engine/core/html/imports/HTMLImportLoader.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 103 matching lines @@
     return StateLoaded;
 }
 
 void HTMLImportLoader::setState(State state)
 {
     if (m_state == state)
         return;
 
     m_state = state;
 
-    if (m_state == StateParsed || m_state == StateError || m_state == StateWritten)
-        m_document->cancelParsing();
+    if (m_state == StateParsed || m_state == StateError || m_state == StateWritten) {
+        if (m_document)
+            m_document->cancelParsing();
+    }
 
     // Since DocumentWriter::end() can let setState() reenter, we shouldn't refer to m_state here.
     if (state == StateLoaded || state == StateError)
         didFinishLoading();
 }
 
 void HTMLImportLoader::didFinishParsing()
 {
     setState(finishParsing());
     setState(finishLoading());
@@ skipping 53 matching lines @@
 {
     visitor->trace(m_controller);
 #if ENABLE(OILPAN)
     visitor->trace(m_imports);
 #endif
     visitor->trace(m_document);
     visitor->trace(m_microtaskQueue);
 }
 
 } // namespace blink