abis.c: Convert early return to assert()
abis.c: Convert early return to assert()

This should simply not happen. If other event is received, something is
terribly wrong, so we should exit immediately instead of silently
failing.

Change-Id: I5f4953be735f526bb8b926979233b3c26ba67ce9

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts