Fix when _stable_1d_sort to work when n >= N

[frankier]

What does this PR do?

Currently if n >= N we will get a "x_pad used before it is defined" type exception.

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[codecov]

Codecov Report

Merging #6177 (0f558bd) into master (1b498d1) will decrease coverage by 2%.
The diff coverage is 89%.

@@           Coverage Diff           @@
##           master   #6177    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         159     159            
  Lines       11380   11361    -19     
=======================================
- Hits        10631   10350   -281     
- Misses        749    1011   +262     

[SkafteNicki]

LGTM