Make SVGTextContentElement methods behave consistently with Firefox and IE

Source/core/svg/SVGTextContentElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Rob Buis <buis@kde.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 105 matching lines @@
 float SVGTextContentElement::getSubStringLength(unsigned charnum, unsigned nchars, ExceptionState& es)
 {
     document()->updateLayoutIgnorePendingStylesheets();
 
     unsigned numberOfChars = getNumberOfChars();
     if (charnum >= numberOfChars) {
         es.throwDOMException(IndexSizeError);
         return 0.0f;
     }
 
+    if (nchars > numberOfChars - charnum)
+        nchars = numberOfChars - charnum;
+
     return SVGTextQuery(renderer()).subStringLength(charnum, nchars);
 }
 
 SVGPoint SVGTextContentElement::getStartPositionOfChar(unsigned charnum, ExceptionState& es)
 {
     document()->updateLayoutIgnorePendingStylesheets();
 
     if (charnum > getNumberOfChars()) {
         es.throwDOMException(IndexSizeError);
         return FloatPoint();
@@ skipping 163 matching lines @@
     SVGElement* element = toSVGElement(renderer->node());
     ASSERT(element);
 
     if (!element->isTextContent())
         return 0;
 
     return toSVGTextContentElement(element);
 }
 
 }