hashindex_compact: fix eval order (check idx before use), fixes #5899

[ThomasWaldmann]

also: fix "off by one" comment

forward port to master.

[codecov-commenter]

Codecov Report

Merging #6818 (2157a35) into master (4e4bfd2) will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6818      +/-   ##
==========================================
+ Coverage   82.55%   82.68%   +0.13%     
==========================================
  Files          39       39              
  Lines       10756    10756              
  Branches     2248     2248              
==========================================
+ Hits         8880     8894      +14     
+ Misses       1351     1339      -12     
+ Partials      525      523       -2     

Impacted Files	Coverage Δ
src/borg/helpers/parseformat.py	90.22% <0.00%> (+0.16%)	⬆️
src/borg/repository.py	82.32% <0.00%> (+0.17%)	⬆️
src/borg/archive.py	81.85% <0.00%> (+0.24%)	⬆️
src/borg/archiver.py	78.57% <0.00%> (+0.30%)	⬆️

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