Bug 752422 - Add null-check for plugin tag. r=bsmedberg

dom/plugins/base/nsPluginHost.cpp

@@ -3170,15 +3170,21 @@ nsPluginHost::StopPluginInstance(nsNPAPIPluginInstance* aInstance)
         nsPluginTag* pluginTag = TagForPlugin(oldestInstance->GetPlugin());
         oldestInstance->Destroy();
         mInstances.RemoveElement(oldestInstance);
+        // TODO: Remove this check once bug 752422 was investigated
+        if (pluginTag) {          
           OnPluginInstanceDestroyed(pluginTag);
         }
       }
+    }
   } else {
     nsPluginTag* pluginTag = TagForPlugin(aInstance->GetPlugin());
     aInstance->Destroy();
     mInstances.RemoveElement(aInstance);
+    // TODO: Remove this check once bug 752422 was investigated
+    if (pluginTag) {      
       OnPluginInstanceDestroyed(pluginTag);
     }
+  }
 
   return NS_OK;
 }