Avoid memory leak if --args is specified multiple times

[cgwalters]

Found by a static analyzer.

bubblewrap-0.4.1/bubblewrap.c:1500: overwrite_var: Overwriting "opt_args_data" in "opt_args_data = load_file_data(the_fd, &data_len)" leaks the storage that "opt_args_data" points to.
 # 1498|              * keep allocated until exit time, since its argv entries get used
 # 1499|              * by the other cases in parse_args_recurse() when we recurse. */
 # 1500|->           opt_args_data = load_file_data (the_fd, &data_len);
 # 1501|             if (opt_args_data == NULL)
 # 1502|               die_with_error ("Can't read --args data");