Fix potential division by zero.

Bug 25351344

Change-Id: Iaf0ce657a3a98c9b70902240340ed5f3f8076281
Reviewed-on: https://swiftshader-review.googlesource.com/4769
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>