avdevice/iec61883: free the private context at the end

Fixes part of ticket #7146.
Signed-off-by: James Almer <jamrial@gmail.com>

libavdevice/iec61883.c

@@ -463,6 +463,7 @@ static int iec61883_close(AVFormatContext *context)
     } else {
         iec61883_dv_fb_stop(dv->iec61883_dv);
         iec61883_dv_fb_close(dv->iec61883_dv);
+        av_freep(&dv->dv_demux);
     }
     while (dv->queue_first) {
         DVPacket *packet = dv->queue_first;