Discovered while working on <rdar://problem/5070334> that many WebView calls will crash if called
        after -[WebView close] has executed because _private->page is deferenced after it's been set to 0. 
        It might be silly/wrong to call these methods after -close, but obviously it shouldn't crash. Made
        each use of _private->page robust against nil-dereferencing.

        * WebView/WebView.mm:
        (-[WebView _loadBackForwardListFromOtherView:]):
        (-[WebView _updateWebCoreSettingsFromPreferences:]):
        (-[WebView _setDashboardBehavior:to:]):
        (-[WebView _dashboardBehavior:]):
        (-[WebView goBack]):
        (-[WebView goForward]):
        (-[WebView goToBackForwardItem:]):
        (-[WebView canGoBack]):
        (-[WebView canGoForward]):
        (-[WebView setTabKeyCyclesThroughElements:]):
        (-[WebView tabKeyCyclesThroughElements]):
        (-[WebView setEditable:]):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20263 268f45cc-cd09-0410-ab3c-d52691b4dbfc