Changes

ranap: Fix decoding of padded ITU X.213 IPv4 TLA in Rab Ass Resp

Simply use the existing omos-iuh library helper to decode the IP
address, which is more robust than existing adhoc code in osmo-sgsn.
The library version already supports decoding ITU X.213 IANA binary IPv4
addresses padded to 20 bytes.

Related: OS#6508
Related: SYS#7119
Change-Id: I9c48b676068574338f60b6ed7ad8b61ba748b52e