Fix missing negative sign with precision 0

[mborland]

Closes: #296

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (95bfc98) to head (87bf578).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #297      +/-   ##
===========================================
+ Coverage    94.86%   94.87%   +0.01%     
===========================================
  Files           74       75       +1     
  Lines         9009     9039      +30     
===========================================
+ Hits          8546     8576      +30     
  Misses         463      463              

Files with missing lines	Coverage Δ
include/boost/charconv/detail/dragonbox/floff.hpp	87.17% <100.00%> (ø)
...lude/boost/charconv/detail/ryu/ryu_generic_128.hpp	92.75% <100.00%> (+0.02%)	⬆️
test/github_issue_296.cpp	100.00% <100.00%> (ø)

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