Avoid division-by-zero in SVGLengthContext::convertValueFromUserUnits

Source/core/svg/SVGLengthContext.cpp

 /*
  * Copyright (C) 2004, 2005, 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * 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.
@@ skipping 231 matching lines @@
 {
     switch (toUnit) {
     case LengthTypeUnknown:
         return 0;
     case LengthTypeNumber:
         return value;
     case LengthTypePercentage: {
         FloatSize viewportSize;
         if (!determineViewport(viewportSize))
             return 0;
+        float dimension = dimensionForLengthMode(mode, viewportSize);
+        if (!dimension)
+            return 0;
         // LengthTypePercentage is represented with 100% = 100.0.
         // Good for accuracy but could eventually be changed.
-        return value * 100 / dimensionForLengthMode(mode, viewportSize);
+        return value * 100 / dimension;
     }
     case LengthTypeEMS:
         return convertValueFromUserUnitsToEMS(computedStyleForLengthResolving(m_context), value);
     case LengthTypeEXS:
         return convertValueFromUserUnitsToEXS(value);
     case LengthTypeREMS:
         return convertValueFromUserUnitsToEMS(rootElementStyle(m_context), value);
     case LengthTypeCHS:
         return convertValueFromUserUnitsToCHS(value);
     case LengthTypePX:
@@ skipping 80 matching lines @@
 
     const SVGSVGElement& svg = toSVGSVGElement(*viewportElement);
     viewportSize = svg.currentViewBoxRect().size();
     if (viewportSize.isEmpty())
         viewportSize = svg.currentViewportSize();
 
     return true;
 }
 
 }