[zlib] Annotate loadchunk with Z_DISABLE_MSAN

There were reports of msan errors in loadchunk internally, similar
to those we've seen in chunkcopy_core before (see first bug).

Bug: 1376033, b/263499981, b/253633142
Change-Id: I27ffc4afa6610d6aa1ce3913f9261f89ad458d69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4136118


Auto-Submit: Hans Wennborg <hans@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1088930}
NOKEYCHECK=True
GitOrigin-RevId: fc816f6f31572bf83c258da9470ce31e98d8d67f