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.

lib/isc/log.c

@@ -1511,6 +1511,9 @@ isc_log_doit(isc_log_t *lctx, isc_logcategory_t *category,
 	lctx->buffer[0] = '\0';
 
 	isc_logconfig_t *lcfg = rcu_dereference(lctx->logconfig);
+	if (lcfg == NULL) {
+		goto unlock;
+	}
 
 	category_channels = ISC_LIST_HEAD(lcfg->channellists[category->id]);