HTMLPreloadScanner should only use valid <base> urls

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 4174 matching lines @@
     }
 }
 
 KURL Document::completeURL(const String& url) const
 {
     return completeURLWithOverride(url, m_baseURL);
 }
 
 KURL Document::completeURLWithOverride(const String& url, const KURL& baseURLOverride) const
 {
+    ASSERT(baseURLOverride.isEmpty() || baseURLOverride.isValid());
+
     // Always return a null URL when passed a null string.
     // FIXME: Should we change the KURL constructor to have this behavior?
     // See also [CSS]StyleSheet::completeURL(const String&)
     if (url.isNull())
         return KURL();
     // This logic is deliberately spread over many statements in an attempt to track down http://crbug.com/312410.
     const KURL* baseURLFromParent = 0;
     bool shouldUseParentBaseURL = baseURLOverride.isEmpty();
     if (!shouldUseParentBaseURL) {
         const KURL& aboutBlankURL = blankURL();
@@ skipping 1492 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif