[zip] Simplify ZipReader::ExtractCurrentEntryToString()

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

Change-Id: I1baa2180fb6987e99d41f607962c71211aac5caa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3461142


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#971606}
NOKEYCHECK=True
GitOrigin-RevId: 398f67e5871d1258e93c719b814b0e112775dd7a