[bsc-nat] ctrlif: use the 'err' label consistently

and propagate -ENOMEM in case we have no memory
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index 2cbb7dc..f39856e 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -1195,14 +1195,12 @@
 		cmd = talloc_zero(bsc, struct ctrl_cmd);
 		if (!cmd) {
 			LOGP(DNAT, LOGL_ERROR, "OOM!\n");
-			return 0;
+			return -ENOMEM;
 		}
 		cmd->type = CTRL_TYPE_ERROR;
 		cmd->id = "err";
 		cmd->reply = "Failed to parse command.";
-		ctrl_cmd_send(&bsc->write_queue, cmd);
-		talloc_free(cmd);
-		return 0;
+		goto err;
 	}
 
 	if (bsc->cfg) {