[zip] Simplify ZipReader::AdvanceToNextEntry

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

Change-Id: Ie54cc0843eb0fa6f0bb89f738a09ddb9ecf58ee5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3450013


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#969222}
NOKEYCHECK=True
GitOrigin-RevId: 3649bcde642cbaa8c7dfc04977f1a6b154403495