Fix handling of trailing 0 in fixed format to_chars with unspecified precision

[mborland]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 98.9%. Comparing base (ee92e74) to head (dccbab7).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/charconv.hpp	58.4%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #793     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          225     226      +1     
  Lines        16940   16949      +9     
  Branches      1806    1807      +1     
=========================================
+ Hits         16739   16748      +9     
  Misses         201     201             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/memcpy.hpp	100.0% <ø> (ø)
test/crash_report_1.cpp	100.0% <100.0%> (ø)
test/test_to_chars.cpp	100.0% <ø> (ø)
include/boost/decimal/charconv.hpp	86.8% <58.4%> (+0.2%)	⬆️

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