We weren't clamping the value when less than zero.

Bug b/37496082

Change-Id: I3170d14a622f8571f0142dd4ce80f96617c7e0aa
Reviewed-on: https://swiftshader-review.googlesource.com/12948Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Casey Dahlin <sadmac@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>