Reviewed by Mark Rowe.

        <rdar://problem/6573916> 
        CrashTracer: [USER] 1 crash in Safari at com.apple.WebKit • WebKit::NetscapePluginInstanceProxy::pluginHostDied + 25.
        
        * Plugins/Hosted/NetscapePluginHostManager.mm:
        (WebKit::NetscapePluginHostManager::instantiatePlugin):
        If we failed to instantiate the plug-in, invalidate the instance proxy.
        
        * Plugins/Hosted/NetscapePluginInstanceProxy.h:
        * Plugins/Hosted/NetscapePluginInstanceProxy.mm:
        (WebKit::NetscapePluginInstanceProxy::invalidate):
        Remove the instance from the plug-in host's set.
        
        (WebKit::NetscapePluginInstanceProxy::destroy):
        Call invalidate().



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@40839 268f45cc-cd09-0410-ab3c-d52691b4dbfc