Reduce truncation in dec_float multiplication

[ckormanyos]

The purpose of this PR is to reduce truncation in cpp_dec_float's multiplication. It is based on some discussion in parts of #368.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e584f4f) 94.1% compared to head (73fb4d3) 94.1%.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #585   +/-   ##
=======================================
  Coverage     94.1%   94.1%           
=======================================
  Files          273     273           
  Lines        28523   28523           
=======================================
  Hits         26835   26835           
  Misses        1688    1688           

Files	Coverage Δ
include/boost/multiprecision/cpp_dec_float.hpp	94.0% <100.0%> (ø)

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[ckormanyos]

Hi Matt (@mborland) I can configure the CodeCov Deltas. I'm a bit surprised that my trivial change led to such a decrease. I'll look into this.

Anyway, we dont want fails due to a small one, two percent delta. I can handle this on the weekend in the CodeCov Properties for this workspace.