Fix buffer sizing calculation for exponents in floff

[mborland]

Closes: #280

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.90%. Comparing base (3554bc1) to head (5aaf6cb).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #281      +/-   ##
===========================================
+ Coverage    94.89%   94.90%   +0.01%     
===========================================
  Files           71       72       +1     
  Lines         9123     9147      +24     
===========================================
+ Hits          8657     8681      +24     
  Misses         466      466              

Files with missing lines	Coverage Δ
include/boost/charconv/detail/dragonbox/floff.hpp	87.24% <100.00%> (-0.03%)	⬇️
test/github_issue_280.cpp	100.00% <100.00%> (ø)

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