gstomxvideoenc: fix subframe output_buffer

Using more than 1 subframes was failing with
frame->output_buffer = NULL
4 jobs for !61 with dab_video_encoder_fix_frame_outbuf in 4 minutes and 51 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Preparation
passed gst indent #1986436

00:00:24

passed manifest #1986435

00:00:27

 
  Build
passed build gst-omx tizonia fedora x86_64 #1986438

00:04:23

passed build gst-omx zynq fedora x86_64 #1986437

00:04:06