Fix memory leak for mpfr/mpc cache (amend 7351e2e)

[skirpichev]

Closes #511

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.61%. Comparing base (224745b) to head (3286b7a).
Report is 44 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
+ Coverage   85.52%   85.61%   +0.08%     
==========================================
  Files          50       50              
  Lines       11656    11655       -1     
  Branches     2202     2202              
==========================================
+ Hits         9969     9978       +9     
+ Misses       1687     1677      -10     

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