Signed integer overflow is undefined behavior in C++.

Change-Id: I12b7507a9624312a615826fd0a1d9cb30b8f8b58
Reviewed-on: https://swiftshader-review.googlesource.com/c/21768Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>