src: enforce assumptions in FIXED_ONE_BYTE_STRING

[tniessen]

These functions are both meant to be used with a null-terminated and thus non-empty sequence of chars. However, there is nothing stopping call sites from passing zero-length sequences, which would certainly not be null-terminated and also would cause an underflow in N - 1. Therefore, this commit

• changes the size N of the array from int to std::size_t,
• ensures that compilation will fail if N = 0, and
• adds a runtime assertion that fails if the N-th char is not \0.

Note that the runtime assertion should be eliminated by any optimizing compiler when given a string literal, which is how these functions are used for the most part (though not exclusively).

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.17%. Comparing base (92102c0) to head (0e394be).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/util.h	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58155      +/-   ##
==========================================
- Coverage   90.18%   90.17%   -0.02%     
==========================================
  Files         630      630              
  Lines      186473   186505      +32     
  Branches    36612    36618       +6     
==========================================
+ Hits       168169   168178       +9     
- Misses      11116    11129      +13     
- Partials     7188     7198      +10     

Files with missing lines	Coverage Δ
src/util.h	89.65% <33.33%> (-1.58%)	⬇️

... and 30 files with indirect coverage changes

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