Mask was set to 32, but should have been 31.
BUG=libyuv:798
TESTED=try bots tested

Change-Id: I6120928873a4a2f1efef907d8e8296ca8c20bb03
Reviewed-on: https://chromium-review.googlesource.com/1054830Reviewed-by: Frank Barchard <fbarchard@chromium.org>