Bug 1430586 - Make sure the NPObjectData map still exists before use. r=jimm

The shutdown path has a way to free the sObjectMap before all plugin references to it have been executed.  It does this by scheduling a DeferNPObjectReleaseRunnable, then shutting down the plugin with PluginInstanceChild::Destroy.  This patch keeps the runnable from failing in that case.

dom/plugins/ipc/PluginModuleChild.cpp

@@ -2375,7 +2375,7 @@ PluginModuleChild::NPN_ReleaseObject(NPObject* aNPObj)
 
     PluginInstanceChild* instance = PluginScriptableObjectChild::GetInstanceForNPObject(aNPObj);
     if (!instance) {
-        NS_ERROR("Releasing object not in mObjectMap?");
+        // The PluginInstanceChild was destroyed
         return;
     }