Fix buffer overrun in link normalisation

[ThisIsFineTM]

This fixes the buffer overrun issue. I changed the raw char* to an iterator and put a couple of bounds checks. First commit updates the normalize_link test which demonstrates the issue with EXPECT. Second commit applies the fix and changes the added tests to be ASSERT.

Closes #439

[veloman-yunkan]

LGTM, but please squash the last two commits into the initial two so that the code change history is cleaner.

[ThisIsFineTM]

@veloman-yunkan squash pushed 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.70%. Comparing base (768cbe5) to head (491caf4).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #446      +/-   ##
==========================================
+ Coverage   27.54%   27.70%   +0.15%     
==========================================
  Files          26       26              
  Lines        2530     2534       +4     
  Branches     1362     1363       +1     
==========================================
+ Hits          697      702       +5     
  Misses       1325     1325              
+ Partials      508      507       -1     

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