Avoid incorrect creation of an Odd<BoxedUint> in invert_mod2k

[andrewwhitehead]

This fixes an issue where Odd::new_unchecked could be used on an even value prior to calling invert_mod_precision. This does not currently create an issue in practice because the result, while incorrect, is reduced to zero bits in that specific case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.83%. Comparing base (ecf51af) to head (e789115).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1257      +/-   ##
==========================================
- Coverage   90.84%   90.83%   -0.01%     
==========================================
  Files         186      186              
  Lines       21673    21670       -3     
==========================================
- Hits        19688    19685       -3     
  Misses       1985     1985              

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