Fix an off-by-one in the powerstore

[Stebalien]

Also fix the test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.95%. Comparing base (12514c1) to head (cfc5564).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
- Coverage   75.18%   74.95%   -0.23%     
==========================================
  Files          70       70              
  Lines        6938     6939       +1     
==========================================
- Hits         5216     5201      -15     
- Misses       1311     1324      +13     
- Partials      411      414       +3     

Files with missing lines	Coverage Δ
internal/powerstore/powerstore.go	61.86% <100.00%> (ø)

... and 4 files with indirect coverage changes