fix hnbgw: RAN_Emu: ranap_unitdata_cb: call only when present

osmo-hnbgw since recently sends own RANAP RESET out to the CN links. So
HNBGW_Tests.ttcn now receives a lot more UnitData messages.

Turns out that tests crash when no UnitData callback is provided.

Currently only osmo-hnbgw and RANAP, i.e. ranap_unitdata_cb, needs this
fix, but also apply the same safeguard to the BSSAP unitdata_cb.

Change-Id: I699a42de88b15f6f47b8feece7639e0dfaf31955
1 file changed