Fix from_chars handling of non-finite 80 and 128-bit values

[mborland]

Closes: #114

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (319b7c6) 86.22% compared to head (6eef3c2) 86.60%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #115      +/-   ##
===========================================
+ Coverage    86.22%   86.60%   +0.38%     
===========================================
  Files           55       55              
  Lines         7891     7990      +99     
===========================================
+ Hits          6804     6920     +116     
+ Misses        1087     1070      -17     

Files	Coverage Δ
...de/boost/charconv/detail/from_chars_float_impl.hpp	65.71% <100.00%> (+7.38%)	⬆️
include/boost/charconv/detail/parser.hpp	74.61% <100.00%> (+3.77%)	⬆️
src/from_chars.cpp	97.77% <100.00%> (+0.63%)	⬆️
test/from_chars_float.cpp	98.58% <100.00%> (+0.07%)	⬆️
test/test_float128.cpp	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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