input: ipaccess: fix possible NULL dereference
input: ipaccess: fix possible NULL dereference

Unlikely to happen, but add assertion for correctness.

Reported by Holger.

Reference: CID 1042352

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