In #12313, it was discovered in a painful way that Sage tries to deallocate twice memory allocated for reduction strategies in pbori.

As this fix is now needed by other tickets like #715, I've put the fix on an independent ticket.

CC: @simon-king-jena @alexanderdreyer

Component: memleak

Keywords: segfault

Author: Jean-Pierre Flori, Simon King

Reviewer: Alexander Dreyer

Merged: sage-5.5.rc1

Issue created by migration from https://trac.sagemath.org/ticket/13746