regression from
commit 3581f1d71a
optimize SharedStringPool::purge() and fix tests
which results in us potentially de-referencing an already de-allocated
OUString object in the first loop in purge().
So switch to a different strategy, which only needs one data structure,
instead of two.
Change-Id: Iaac6beda48459643afdb7b14ce7d39d68a93339c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/95226
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
5af636c502