Changes

l1sap: make send_ul_rtp_packet_hrdata() NULL-safe

lchan->abis_ip.rtp_socket is NULL before the BSC indicates the RTP
parameters using the CRCX/MDCX procedures.  send_ul_rtp_packet()
does check lchan->abis_ip.rtp_socket against NULL before calling
osmo_rtp_send_frame_ext(), so should send_ul_rtp_packet_hrdata().

Take a chance to make send_ul_rtp_packet_hrdata() more consistent
with the send_ul_rtp_packet() by reordering code a bit.

Change-Id: I05d88a739dc54ca68e50d20b2d0d0b097ae8ca4a
Fixes: 59686cd27 ("CSD: implement half-rate modes correctly")
Related: OS#6618