Assertion failure in http/tests/xmlhttprequest/failed-auth.html.

        I couldn't find out why this only started to happen now, but it was incorrect to check
        persistence of a credential returned by CredentialStorage::get() without checking that it
        was non-null. When there is no credential for the protection space in storage, get()
        returns a new object, and Credentil constructor doesn't initialize m_persistence.

        * platform/network/mac/ResourceHandleMac.mm:
        (-[WebCoreSynchronousLoader connection:didReceiveAuthenticationChallenge:]): Moved the
        assertion after credential null check.
        (WebCore::ResourceHandle::didReceiveAuthenticationChallenge): Added the same persistence
        assertion, matching sync code (and CF one, as well).



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@48918 268f45cc-cd09-0410-ab3c-d52691b4dbfc