libs: encoder: h265: bail if nal unit type fails
libs: encoder: h265: bail if nal unit type fails

Bail out if the NAL unit type is not recognized.

https://bugzilla.gnome.org/show_bug.cgi?id=778782

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