common: Fix check_after_deref in cockpit_certificate_locate()
common: Fix check_after_deref in cockpit_certificate_locate()

There is no need to check `error` in the condition, as the code above
already dereferences it. Spotted by Coverity.

garrett/cockpit,garrett/cockpit,garrett/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit