lang: add null pointer check to arith_parse_left_paren

this fixes a crash that occurred when the call-operator was used on a lone ident in an expression.

lang/ast/expr/arith.c

@@ -598,7 +598,7 @@ struct token_parse_result arith_parse_left_paren(
 		struct ivy_ast_node *msg
 			= b_unbox(struct ivy_ast_node, msg_entry, n_entry);
 
-		if (msg->n_type != IVY_AST_MSG) {
+		if (!msg || msg->n_type != IVY_AST_MSG) {
 			/* this is not a complex message, it's probably a
 			 * call-operator */
 			goto not_complex_msg;