archive-zip: widen `zlib_deflate_raw()`'s maxsize local to `size_t`

archive-zip: widen `zlib_deflate_raw()`'s maxsize local to `size_t`

Prep for the upcoming `git_deflate_bound()` widening to `size_t`: the
local that catches its return needs to be `size_t` too, otherwise the
widening would introduce a silent Windows narrowing here. No semantic
effect with the current unsigned-long-returning `git_deflate_bound()`
(`size_t == unsigned long` on this caller's platforms today).

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>