which only recent versions of MSVC (from 15.8.6) seem to spot.
It resulted in PythonTest_sw_python failures with "vector
iterators incompatible" assertion on the
    dest != maDestructedListeners.end()
check

ever since
    commit 4a290888
    Date:   Mon Oct 1 14:31:00 2018 +0200
    SvtBroadcaster unify the normal and PrepareForDestruction paths

Although this code appears to always have been wrong, it is just
that it is being hit much more often now.

Change-Id: I45d4f2ceef4ddf19c205702645a41363eea93f21
Reviewed-on: https://gerrit.libreoffice.org/61397
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>