Fixes a small bug in xxhash.

Bug: skia:7647
Change-Id: Ibd0bd8dc4febfe585e257786d9683ea09c94ec6a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2376718Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>