reset VisibleSelection at the Undo command.

Source/core/editing/VisibleSelection.cpp

 /*
  * Copyright (C) 2004, 2005, 2006 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 753 matching lines @@
 
 void VisibleSelection::trace(Visitor* visitor)
 {
     visitor->trace(m_base);
     visitor->trace(m_extent);
     visitor->trace(m_start);
     visitor->trace(m_end);
     visitor->trace(m_changeObserver);
 }
 
+static bool isValidPosition(const Position& position)
+{
+    if (!position.inDocument())
+        return false;
+
+    if (position.anchorType() != Position::PositionIsOffsetInAnchor)
+        return true;
+
+    if (position.offsetInContainerNode() < 0)
+        return false;
+
+    const unsigned offset = static_cast<unsigned>(position.offsetInContainerNode());
+    const unsigned nodeLength = position.anchorNode()->lengthOfContents();
+    return offset <= nodeLength;
+}
+
+void VisibleSelection::validatePositionsIfNeeded()
+{
+    if (!isValidPosition(m_base) || !isValidPosition(m_extent) || !isValidPosition(m_start) || !isValidPosition(m_end))
+        validate();
+}
+
 #ifndef NDEBUG
 
 void VisibleSelection::debugPosition() const
 {
     fprintf(stderr, "VisibleSelection ===============\n");
 
     if (!m_start.anchorNode())
         fputs("pos:   null", stderr);
     else if (m_start == m_end) {
         fprintf(stderr, "pos:   %s ", m_start.anchorNode()->nodeName().utf8().data());
@@ skipping 51 matching lines @@
     sel.showTreeForThis();
 }
 
 void showTree(const WebCore::VisibleSelection* sel)
 {
     if (sel)
         sel->showTreeForThis();
 }
 
 #endif