nat: Check for the access list in set command as well

I omitted the check as this was already done by the verify
function for this command. Please Coverity and do the check
again even if it is not necessary. I begin to doubt the
usage of a "dedicated" verify method as well.

Silences: Coverity CID 1293150
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c b/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c
index 54e1dc4..4b59b40 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c
@@ -404,11 +404,18 @@
 static int set_net_cfg_acc_cmd(struct ctrl_cmd *cmd, void *data)
 {
 	const char *access_name = extract_acc_name(cmd->variable);
-	struct bsc_nat_acc_lst *acc = bsc_nat_acc_lst_find(g_nat, access_name);
+	struct bsc_nat_acc_lst *acc;
 	struct bsc_nat_acc_lst_entry *entry;
 	const char *value = cmd->value;
 	int rc;
 
+	/* Should have been caught by verify_net_cfg_acc_cmd */
+	acc = bsc_nat_acc_lst_find(g_nat, access_name);
+	if (!acc) {
+		cmd->reply = "Access list not found";
+		return CTRL_CMD_ERROR;
+	}
+
 	entry = bsc_nat_acc_lst_entry_create(acc);
 	if (!entry) {
 		cmd->reply = "OOM";