microtar from mem.. -6 bad checksum

[otse]

Hi. Nice C thing. I wrote my mem_ variants from the file_ variants.

The tar->stream was only used in these four callbacks, so I thought this would work, but I get a -6 bad checksum.

static int mem_write(mtar_t *tar, const void *data, unsigned size)
{
	//unsigned res = fwrite(data, 1, size, (FILE *)tar->stream);
	//return (res == size) ? MTAR_ESUCCESS : MTAR_EWRITEFAIL;
	return MTAR_EWRITEFAIL;
}

static int mem_read(mtar_t *tar, void *data, unsigned size)
{
	unsigned char *buf = (unsigned char *)tar->stream;
	data = &buf[tar->pos];
	return MTAR_ESUCCESS;
}

static int mem_seek(mtar_t *tar, unsigned offset)
{
	return MTAR_ESUCCESS;
}

static int mem_close(mtar_t *tar)
{
	return MTAR_ESUCCESS;
}

int mtar_open_mem(mtar_t *tar, void *data)
{
	int err;
	mtar_header_t h;

	// Init tar struct and functions
	memset(tar, 0, sizeof(*tar));
	tar->write = mem_write;
	tar->read = mem_read;
	tar->seek = mem_seek;
	tar->close = mem_close;

	tar->stream = data;

	err = mtar_read_header(tar, &h);
	if (err != MTAR_ESUCCESS)
	{
		mtar_close(tar);
		return err;
	}

	// Return ok
	return MTAR_ESUCCESS;
}```