Zlib patch: prevent uninitialized use of state->check

Zlib patch: prevent uninitialized use of state->check

This CL fixes a security bug in zlib. It was reported upstream long ago
and the testcase was shared upstream but it's yet unsolved. As a fix,
state->check is set to the same value as the adler32 of an empty string.

Upstream bug: madler/zlib#245

Bug: chromium:697481 https://crbug.com/697481
Reviewed-on: https://chromium-review.googlesource.com/601193
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>