sysmobts: Check mgr->calib.bts_conn for NULL
sysmobts: Check mgr->calib.bts_conn for NULL

Check the right variable for NULL.

Fixes: CID 1262214

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