Avoid division-by-zero in SVGLengthContext::convertValueFromUserUnits

Source/core/svg/SVGLengthContext.cpp

Index: Source/core/svg/SVGLengthContext.cpp
diff --git a/Source/core/svg/SVGLengthContext.cpp b/Source/core/svg/SVGLengthContext.cpp
index b64cfd4b81ad9e7f96af5aeb5536c1d5ebee22b1..34e30e9b366908804ec16f133a72d840e047500b 100644
--- a/Source/core/svg/SVGLengthContext.cpp
+++ b/Source/core/svg/SVGLengthContext.cpp
@@ -249,9 +249,12 @@ float SVGLengthContext::convertValueFromUserUnits(float value, SVGLengthMode mod
         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);