Fix RadixNode.matchAt

[johnynek]

followup to #352 to fix an issue only noticed on second review.

Added a test on Parser first that failed, then fixed the bug.

[codecov-commenter]

Codecov Report

Merging #362 (e04b9c9) into main (828ff4b) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
+ Coverage   96.63%   96.71%   +0.08%     
==========================================
  Files           9        9              
  Lines        1128     1128              
  Branches       99      111      +12     
==========================================
+ Hits         1090     1091       +1     
+ Misses         38       37       -1     

Impacted Files	Coverage Δ
...e/shared/src/main/scala/cats/parse/RadixNode.scala	95.31% <100.00%> (ø)
core/shared/src/main/scala/cats/parse/Parser.scala	96.69% <0.00%> (+0.11%)	⬆️

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