Optimize canonicalize_pathname function

[rmisev]

Use modified_value only when prepending "/-" to the path name value.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.62%. Comparing base (89cdfcd) to head (015f668).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/upa/urlpattern.h	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
- Coverage   99.65%   99.62%   -0.03%     
==========================================
  Files          21       21              
  Lines        3734     3739       +5     
  Branches      663      664       +1     
==========================================
+ Hits         3721     3725       +4     
- Misses         13       14       +1     

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