fix: dev: Check if logconfig is NULL before using it in isc_log_doit()

Check if 'lctx->logconfig' is NULL before using it in isc_log_doit(),
because it's possible that isc_log_destroy() was already called, e.g.
when a 'call_rcu' function wants to log a message during shutdown.

Closes #4842

Merge branch '4842-shutdown-crash-in-isc_log_doit' into 'main'

Closes #4842

See merge request isc-projects/bind9!9297