[zlib] Zero-initialize the window used for deflation

Otherwise MSan complains about use-of-uninitialized values in the window.
This happens in both regular deflate's longest_match and deflate_rle.

Before crrev.com/822755 we used to suppress those reports, but it seems
better to fix it properly. That will also allow us to catch other
potential issues with MSan in these functions.

The instances of this that we've seen only reproduce with
fill_window_sse(), not with the regular fill_window() function. Since
the former doesn't exist in upstream zlib, I'm not planning to send this
patch upstream.

Bug: 1137613, 1144420
Change-Id: I2b1801cd2a63fef48a0072b2d2c8fc1f8a7bb920
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517520


Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823845}
GitOrigin-RevId: 962cbbe81708214ff8e14e2bc8a07271cb15f1b9