vpxenc: Unref frame on ERROR
vpxenc: Unref frame on ERROR

All code paths for handle_frame() must somehow take ownership of the frame, be
it by actually unreffing, forwarding the frame elsewhere or storing it for
later.

http://bugzilla.gnome.org/show_bug.cgi?id=760666

pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good