Reviewed by Dan Bernstein

        <rdar://problem/6563402> - Workaround a case where ::didReceiveData() might be called on a ResourceHandleClient 
        before ::didReceiveResponse().

        No layout test, as we don't have a concrete way to reproduce these conditions.  To that effect, I've added an ASSERT
        to catch the conditions before the workaround takes effect in case any WebKit Debug-build developer runs across
        this case and can come up with something more reproducible.

        * loader/MainResourceLoader.cpp:
        (WebCore::MainResourceLoader::didReceiveData): If the response is null create a dummy response.  This is much 
          like the FrameLoader::init() response that is used for initial document creation.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@40745 268f45cc-cd09-0410-ab3c-d52691b4dbfc