Decoder bugfix - possible lock when using FEC

* when using FEC and first frame comes corrupted, we do not write back
  frame (which we just obtained from reconfigured display)
* TODO: refactorize whole decoder/reconfigure stuff (!)

ultragrid/src/rtp/decoders.c

@@ -815,6 +815,7 @@ packet_restored:
 
                         frame = reconfigure_decoder(decoder, decoder->received_vid_desc,
                                         frame);
+                        pbuf_data->frame_buffer = frame;
                 }
 
                 if(!frame) {