Fix non-finite return value of abs(complex(float128))

[mborland]

Closes: #759

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.2%. Comparing base (965194a) to head (52b591b).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #760     +/-   ##
=========================================
+ Coverage     96.2%   96.2%   +0.1%     
=========================================
  Files          301     301             
  Lines        29462   29462             
=========================================
+ Hits         28328   28332      +4     
+ Misses        1134    1130      -4     

Files with missing lines	Coverage Δ
include/boost/multiprecision/float128.hpp	84.1% <ø> (ø)

... and 2 files with indirect coverage changes

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[cffk]

Thanks!