NTILE must return an error when parameter is not stable

Ntile must only work for deterministic cases.

If we have a query such as

select ntile( n ) over (partition by c1 order by c2):

n must provide a stable value for every individual partition, or we should return an error.

Currently we compute

 ntile(<current_value_of_n>) 

, given the current row number in the partition. This leads to erroneous results.