Fix first character from_chars integer pattern matching

[mborland]

Closes: #188

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.39%. Comparing base (100473b) to head (6bab2a7).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #189   +/-   ##
========================================
  Coverage    93.38%   93.39%           
========================================
  Files           66       66           
  Lines         8420     8432   +12     
========================================
+ Hits          7863     7875   +12     
  Misses         557      557           

Files	Coverage Δ
.../boost/charconv/detail/from_chars_integer_impl.hpp	100.00% <100.00%> (ø)
test/from_chars.cpp	100.00% <100.00%> (ø)
test/quick.cpp	100.00% <ø> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 100473b...6bab2a7. Read the comment docs.