Fix calculation of resultant digits to keep us in the range of uint128

[mborland]

Closes: #802

[libbooze]

Not sure adding this as test is useful.
Result was always correct afaik.
This is hard to test but if you want one way to test this is to introduce detail:: helper that checks if 2 numbers can be multiplied within range of u128, and then test that helper.
Might be an overkill, but it is basically just extracting

    const auto lhs_dig {detail::num_digits(lhs_sig)};
    const auto rhs_dig {detail::num_digits(rhs_sig)};
    return lhs_dig + rhs_dig <= std::numeric_limits<uint128>::digits10;
    

into bool returning helper function.

[mborland]

It's a good reproducer for using the debugger. I'm not super worried about the test in this case since the special functions test suites will for sure hit both paths.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (30ed86c) to head (35ada3d).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #812     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          228     229      +1     
  Lines        16978   16987      +9     
  Branches      1810    1809      -1     
=========================================
+ Hits         16787   16796      +9     
  Misses         191     191             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/mul_impl.hpp	94.9% <100.0%> (ø)
test/github_issue_802.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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