libs: egl: utils: mark context as wrapped when it is
libs: egl: utils: mark context as wrapped when it is

The returning egl context may be null, so we should check the
return value.

https://bugzilla.gnome.org/show_bug.cgi?id=795391

ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi