tx_power.c: fix potential array out-of-bounds access
tx_power.c: fix potential array out-of-bounds access

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