Fixed condition too strict in glBindSampler

Fixed condition too strict in glBindSampler

glBindSampler's sampler check was too strict, as the
sampler needed to exist, but not necessarily to have
been allocated yet, so the condition was fixed.

Change-Id: Ia8301028dbf35efa16bfc6cb74a69e105d8a505d
Reviewed-on: https://swiftshader-review.googlesource.com/3965
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>