Attach the RTP access output grabber object.

Fix #1486
Signed-off-by: Rémi Denis-Courmont <rem@videolan.org>

modules/stream_out/rtp.c

@@ -1677,5 +1677,6 @@ static sout_access_out_t *GrabberCreate( sout_stream_t *p_stream )
     p_grab->p_sys       = (sout_access_out_sys_t *)p_stream;
     p_grab->pf_seek     = NULL;
     p_grab->pf_write    = AccessOutGrabberWrite;
+    vlc_object_attach( p_grab, p_stream );
     return p_grab;
 }