Fix an MQ parsing related crash

Source/core/css/MediaQueryExp.cpp

 /*
  * CSS Media Query
  *
  * Copyright (C) 2006 Kimmo Kinnunen <kimmo.t.kinnunen@nokia.com>.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  * Copyright (C) 2013 Apple 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:
@@ skipping 196 matching lines @@
 
 MediaQueryExp::MediaQueryExp(const String& mediaFeature, PassRefPtrWillBeRawPtr<CSSValue> value)
     : m_mediaFeature(mediaFeature)
     , m_value(value)
 {
 }
 
 // FIXME - create should not return a null.
 PassOwnPtrWillBeRawPtr<MediaQueryExp> MediaQueryExp::create(const String& mediaFeature, CSSParserValueList* valueList)
 {
+    ASSERT(!mediaFeature.isNull());
+
     RefPtrWillBeRawPtr<CSSValue> cssValue;
     bool isValid = false;
     String lowerMediaFeature = attemptStaticStringCreation(mediaFeature.lower());
 
     // Create value for media query expression that must have 1 or more values.
     if (valueList && valueList->size() > 0) {
         if (valueList->size() == 1) {
             CSSParserValue* value = valueList->current();
+            ASSERT(value);
 
             if (featureWithCSSValueID(lowerMediaFeature, value)) {
                 // Media features that use CSSValueIDs.
                 cssValue = CSSPrimitiveValue::createIdentifier(value->id);
                 if (!featureWithValidIdent(lowerMediaFeature, toCSSPrimitiveValue(cssValue.get())->getValueID()))
                     cssValue.clear();
             } else if (featureWithValidDensity(lowerMediaFeature, value)) {
                 // Media features that must have non-negative <density>, ie. dppx, dpi or dpcm.
                 cssValue = CSSPrimitiveValue::create(value->fValue, (CSSPrimitiveValue::UnitTypes) value->unit);
             } else if (featureWithValidPositiveLength(lowerMediaFeature, value)) {
@@ skipping 60 matching lines @@
     if (m_value) {
         result.append(": ");
         result.append(m_value->cssText());
     }
     result.append(")");
 
     return result.toString();
 }
 
 } // namespace