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/3965Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>