Fix scientific/fixed format crossover point and overzealous boundary protection

[mborland]

Closes: #282

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.92%. Comparing base (4a6e906) to head (fb9008f).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #283      +/-   ##
===========================================
+ Coverage    94.90%   94.92%   +0.02%     
===========================================
  Files           72       73       +1     
  Lines         9147     9186      +39     
===========================================
+ Hits          8681     8720      +39     
  Misses         466      466              

Files with missing lines	Coverage Δ
src/to_chars.cpp	92.94% <100.00%> (ø)
src/to_chars_float_impl.hpp	83.45% <100.00%> (+0.05%)	⬆️
test/github_issue_282.cpp	100.00% <100.00%> (ø)

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