Fix decimal128_fast scalblnd implementation

[mborland]

Closes: #805

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (437d912) to head (1b8a11b).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #821     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          230     231      +1     
  Lines        17055   17084     +29     
  Branches      1808    1810      +2     
=========================================
+ Hits         16864   16894     +30     
+ Misses         191     190      -1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_fast.hpp	98.4% <100.0%> (+0.1%)	⬆️
test/github_issue_805.cpp	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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