lang: codegen: fix segfault when serialising selector with unnamed args

lang/codegen/msg.c

@@ -122,9 +122,19 @@ static void serialise_selector(
 	b_queue_foreach (&it, &sel->n_arg_labels) {
 		struct ivy_token *arg
 			= b_unbox(struct ivy_token, it.entry, t_entry);
+
+		if (arg->t_type != IVY_TOK_IDENT) {
+			b_stringstream_addf(out, "0");
+			continue;
+		}
+
 		const char *label = arg->t_str;
 
+		if (label) {
 			b_stringstream_addf(out, "%zu%s", strlen(label), label);
+		} else {
+			b_stringstream_addf(out, "0");
+		}
 	}
 
 	b_stringstream_add(out, "E");