vaapipluginbase: Fix structure memory leak
vaapipluginbase: Fix structure memory leak

config structure is not being freed in all cases

https://bugzilla.gnome.org/show_bug.cgi?id=762172

01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi