Bug fix: StyledMarkupSerializer's m_lastClosed can be null

Source/core/editing/StyledMarkupSerializer.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009, 2010, 2011 Google Inc. All rights reserved.
  * Copyright (C) 2011 Igalia S.L.
  * Copyright (C) 2011 Motorola Mobility. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 255 matching lines @@
 template<typename Strategy>
 StyledMarkupTraverser<Strategy>::StyledMarkupTraverser(StyledMarkupAccumulator* accumulator, Node* lastClosed)
     : m_accumulator(accumulator)
     , m_lastClosed(lastClosed)
     , m_wrappingStyle(nullptr)
 {
     if (!m_accumulator) {
         ASSERT(!m_lastClosed);
         return;
     }
-    ASSERT(m_lastClosed);
+    if (!m_lastClosed)
+        return;
     ContainerNode* parent = Strategy::parent(*m_lastClosed);
     if (!parent)
         return;
     m_wrappingStyle = EditingStyle::wrappingStyleForSerialization(parent, shouldAnnotate());
 }
 
 template<typename Strategy>
 Node* StyledMarkupTraverser<Strategy>::traverse(Node* startNode, Node* pastEnd)
 {
     WillBeHeapVector<RawPtrWillBeMember<ContainerNode>> ancestorsToClose;
@@ skipping 198 matching lines @@
     if (element.isHTMLElement() && shouldAnnotate())
         inlineStyle->mergeStyleFromRulesForSerialization(&toHTMLElement(element));
 
     return inlineStyle;
 }
 
 template class StyledMarkupSerializer<EditingStrategy>;
 template class StyledMarkupSerializer<EditingInComposedTreeStrategy>;
 
 } // namespace blink