lchan: Fix crashes when the specified lchan can not be found
lchan: Fix crashes when the specified lchan can not be found

gsm_lchan_name will crash if the lchan is NULL. Introduce an error_report
method that will do the right thing in the future and report the error.

telenoobie/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts