Avoid to insert TAB before HTML element

Source/core/editing/InsertTextCommand.cpp

 /*
  * Copyright (C) 2005 Apple Computer, 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 211 matching lines @@
             applyStyle(typingStyle.get());
     }
 
     if (!m_selectInsertedText)
         setEndingSelection(VisibleSelection(endingSelection().end(), endingSelection().affinity(), endingSelection().isDirectional()));
 }
 
 Position InsertTextCommand::insertTab(const Position& pos)
 {
     Position insertPos = VisiblePosition(pos, DOWNSTREAM).deepEquivalent();
+    if (insertPos.isNull())
+        return pos;
 
     Node* node = insertPos.containerNode();
     unsigned offset = node->isTextNode() ? insertPos.offsetInContainerNode() : 0;
 
     // keep tabs coalesced in tab span
     if (isTabSpanTextNode(node)) {
         RefPtrWillBeRawPtr<Text> textNode = toText(node);
         insertTextIntoNode(textNode, offset, "\t");
         return Position(textNode.release(), offset + 1);
     }
@@ skipping 17 matching lines @@
                 splitTextNode(textNode, offset);
             insertNodeBefore(spanNode, textNode.release());
         }
     }
 
     // return the position following the new tab
     return lastPositionInNode(spanNode.get());
 }
 
 }