archive_string: Check values before casts

archive_string: Check values before casts

The size_t to int conversion is especially required on Windows systems
to support their int-based functions. These variables should be properly
checked before casts. This avoids integer truncations with large
strings.

I prefer size_t over int for sizes and adjusted variables to size_t
where possible to avoid casts.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>