and fix leak of HelpListener_Impl in SfxHelpWindow_Impl

Change-Id: I7450be10c8deaf63b7c7f1f4359b4eaf0083bdd4
Reviewed-on: https://gerrit.libreoffice.org/54451Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>