Add fuzzing to snprintf and fix overflow

[mborland]

Closes: #502

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 98.7%. Comparing base (ac3edde) to head (3e6412b).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #503     +/-   ##
=========================================
- Coverage     98.7%   98.7%   -0.0%     
=========================================
  Files          179     179             
  Lines        11411   11418      +7     
=========================================
+ Hits         11262   11267      +5     
- Misses         149     151      +2     

Files	Coverage Δ
include/boost/decimal/cstdio.hpp	92.9% <95.7%> (+1.9%)	⬆️
include/boost/decimal/charconv.hpp	94.9% <50.0%> (-1.0%)	⬇️

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