Fix validation of texture level.

Early out checks for zero size were skipping the validation.

Change-Id: I1d8173146117be5a7a3330cf6eae46c2ef8231ff
Reviewed-on: https://swiftshader-review.googlesource.com/5093
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>