We could overflow when casting from max unsigned int to integer, and
return a negative mask.

See dEQP functional.state_query.stencil_value_mask_getfloat

Change-Id: I59189a40b662ad9d8ded79cdfeded923b98f0b85
Reviewed-on: https://chromium-review.googlesource.com/181780Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Commit-Queue: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>