Reuse r.ptr instead of using strlen to get return size

[pps83]

No description provided.

[mborland]

Thanks. I kicked off the CI run

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.5%. Comparing base (17cfe8b) to head (999ce9f).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1339     +/-   ##
=========================================
- Coverage     98.5%   98.5%   -0.0%     
=========================================
  Files          282     282             
  Lines        22551   22550      -1     
  Branches      1960    1960             
=========================================
- Hits         22200   22198      -2     
- Misses         351     352      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/string.hpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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