Fix race on x86_cpu_enable_ssse3

adler32_z sets this value through cpu_check_features when called like
'adler32(0, Z_NULL, 0)'. However, the value is used in a branch
condition earlier without first checking that 'buf != Z_NULL'. Re-order
the earlier predicate to only read from x86_cpu_enable_ssse3 when
'buf != Z_NULL'.

Change-Id: I43c228d878721c566c793247acdef462d463740e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2659089


Commit-Queue: Ben Wagner <bungeman@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
Cr-Commit-Position: refs/heads/master@{#848802}
GitOrigin-RevId: fa596d830e98b0f40c3cc72269edb69fc810c175