BUG: Fix incorrect comparison in assert
BUG: Fix incorrect comparison in assert

The assert was suppose to check if the pixel id would be a valid
index and include 0 as valid.

SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging