ipa_bts_id_resp(): handle non-existing members of ipacces_unit
ipa_bts_id_resp(): handle non-existing members of ipacces_unit

Just because the IPA server is requesting certain identities via the CCM
protocol, doesn't mean that the client actually has those fields
available and initialized in struct ipaccess_unit.

Instad of segfaulting the client, let's check if the respective
identities are known.  If yes, send them.  If not, send the empty string
for that particular identity.

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis