Segmentation fault when destroy a `String` with `default`

[kioz-wang]

Look at the code:

test "Bug, destroy default String" {
    const cmd = Command.new("cmd").arg(Arg.posArg("pos", String).default("hello"));
    var it = try TokenIter.initList(&.{}, .{});
    const args = try cmd.parseFrom(&it, testing.allocator);
    cmd.destroy(&args, testing.allocator);
}

It would raise segmentation fault because that allocator attempts to free the memory of "hello".

[kioz-wang]

const String = []const u8