Safeguard for plugin object

On iOS and Android, without the safeguard, these runtime errors can occur:

    JS: assets/jsb/jsb_pluginx.js:1:ReferenceError: plugin is not defined
    JS: assets/jsb/jsb_pluginx.js:11:TypeError: plugin.ProtocolAds is undefined

This safeguard fixes the errors.
Merge pull request #42 from NatWeiss/patch-4

Safeguard for plugin object