span: fix edge cases

[biojppm]

No description provided.

[codecov]

Codecov Report

Merging #49 (99f2252) into master (19ec7fb) will increase coverage by 0.13%.
The diff coverage is 99.85%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   97.98%   98.11%   +0.13%     
==========================================
  Files          49       49              
  Lines        8233     8861     +628     
==========================================
+ Hits         8067     8694     +627     
- Misses        166      167       +1     

Impacted Files	Coverage Δ
src/c4/substr.hpp	98.82% <98.85%> (-0.07%)	⬇️
src/c4/span.hpp	97.91% <100.00%> (+0.02%)	⬆️
test/test_span.cpp	100.00% <100.00%> (ø)
test/test_substr.cpp	99.14% <100.00%> (+0.14%)	⬆️

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