deflateCopy(): Fix allocation and copying of pending_buf in LIT_MEM mode

This applies the fix from https://github.com/madler/zlib/pull/900 and
adds a test.

Bug: 1513368
Change-Id: I38025c66e1ef2420c726406ddefe6c728bdb6458
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5139141


Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1239920}
NOKEYCHECK=True
GitOrigin-RevId: 5672e5e75c159ba1742f8cfd784d294cff21c597