[zip] Simplify ZipReader::OpenInternal

BUG=chromium:1295127
TEST=autoninja -C out/Default zlib_unittests && out/Default/zlib_unittests

Change-Id: I75d5598be2c413e4974cc3ee57e7850de89f2711
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3449731


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#969256}
NOKEYCHECK=True
GitOrigin-RevId: fde2706f6c5aa059ba1985211e4b2f5b63b5b1b8