Avoid double counting digits in normalization process

[mborland]

Closes: #1111

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (4d0c7e2) to head (c88023f).
⚠️ Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1125     +/-   ##
=========================================
+ Coverage     99.4%   99.4%   +0.1%     
=========================================
  Files          246     246             
  Lines        16778   16777      -1     
  Branches      1812    1812             
=========================================
+ Hits         16671   16672      +1     
+ Misses         107     105      -2     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.5% <100.0%> (ø)
include/boost/decimal/decimal32_t.hpp	96.5% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	97.2% <100.0%> (ø)
include/boost/decimal/detail/fenv_rounding.hpp	98.2% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/normalize.hpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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