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

ext/vpx/gstvpxenc.c

@@ -1903,6 +1903,7 @@ gst_vpx_enc_handle_frame (GstVideoEncoder * video_encoder,
     GST_ELEMENT_ERROR (encoder, LIBRARY, ENCODE,
         ("Failed to encode frame"), ("%s", gst_vpx_error_name (status)));
     gst_video_codec_frame_set_user_data (frame, NULL, NULL);
+    gst_video_codec_frame_unref (frame);
 
     return GST_FLOW_ERROR;
   }