blind fix since I can't reproduce. But I can see from the reported
threads that two different threads are entering SfxItemPool::Put and
consequently the debug runtime throws an iterator exception.

Change-Id: Idaa6a93b134edc49bda8cd7876bb216d5aec7037
Reviewed-on: https://gerrit.libreoffice.org/73272
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>