When using gst_rtcp_packet_fb_set_fci_length to set the length of the
FCI in a feedback message, the buffer becomes 8 bytes longer than expected.

The length that is calculated and written to the packet is later used to increment
the buffer length.

from gstrtcpbuffer.c:
gst_rtcp_packet_fb_set_fci_length (GstRTCPPacket * packet, guint16 wordlen)
  .........
  data = packet->rtcp->map.data + packet->offset + 2;
  wordlen += 2;                                            <= wordlen increased by 2
  GST_WRITE_UINT16_BE (data, wordlen);

  packet->rtcp->map.size += wordlen * 4;  <= wordlen used to increase buffer size

  return TRUE;
}

The resulting buffer is therefore 8 bytes too big, because the non-FCI part of the 
packet is already accounted for when created in gst_rtcp_buffer_add_packet:
...
case GST_RTCP_TYPE_RTPFB:
      len = 12;
      break;
case GST_RTCP_TYPE_PSFB:
      len = 12
...
rtcp->map.size += len;
...