Bug fix: StyledMarkupSerializer's m_lastClosed can be null

Source/core/editing/StyledMarkupSerializer.cpp

Index: Source/core/editing/StyledMarkupSerializer.cpp
diff --git a/Source/core/editing/StyledMarkupSerializer.cpp b/Source/core/editing/StyledMarkupSerializer.cpp
index 8953d312dbbba2fd6da6fa9f3cf2f14df7c3530a..eb1f09e6e0ac6aebd13d034ea13c657260062cc3 100644
--- a/Source/core/editing/StyledMarkupSerializer.cpp
+++ b/Source/core/editing/StyledMarkupSerializer.cpp
@@ -273,7 +273,8 @@ StyledMarkupTraverser<Strategy>::StyledMarkupTraverser(StyledMarkupAccumulator*
         ASSERT(!m_lastClosed);
         return;
     }
-    ASSERT(m_lastClosed);
+    if (!m_lastClosed)
+        return;
     ContainerNode* parent = Strategy::parent(*m_lastClosed);
     if (!parent)
         return;