fix: invalid data successfully decoded when using altchars

[mayeut]

When a slice used for alphabet translation ends with valid padding, the excess data after padding is not detected.

[codspeed-hq]

Merging this PR will not alter performance

✅ 48 untouched benchmarks

---

_{Comparing altchars-limit (c6a755c) with master (a7925c4)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (a7925c4) to head (c6a755c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1040   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files          11       11           
  Lines        1914     1927   +13     
  Branches      306      307    +1     
=======================================
+ Hits         1906     1919   +13     
  Partials        8        8           

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