Treat linethickness < 0 as zero.

closes w3c/mathml#195

Author: fred-wang

index.html

@@ -2282,6 +2282,7 @@ <h4>Fractions <code>&lt;mfrac&gt;</code></h4>
             If the attribute is absent or has an invalid value,
             <a>FractionRuleThickness</a> is used as the default
             value. A percentage is interpreted relative to that default value.
+            A negative value is interpreted as 0.
           </p>
           <p>
             The <code>&lt;mfrac&gt;</code> element sets