fix fullscreen handler if adsManager uninitialized
fix fullscreen handler if adsManager uninitialized

We don't call for ads on every video, thus there is no call to requestAds() and subsequently no adsManger is created. This fix checks for an existing adsManager when handling the fullscreen-change event.

tv2/videojs-ima,fem-inc/videojs-ima,ypavlotsky/videojs-ima,googleads/videojs-ima,shawnbuso/videojs-ima,googleads/videojs-ima,shawnbuso/videojs-ima,tape-tv/videojs-ima,valse/videojs-ima,fem-inc/videojs-ima,tv2/videojs-ima,valse/videojs-ima,goskipstone/videojs-ima,ypavlotsky/videojs-ima,goskipstone/videojs-ima,tape-tv/videojs-ima