Accessibility: Avoid immediate crash in Impress

When Accessibility is active Impress immediately crashes. We need to check that
mpParent isn't NULL before we try to call it's methods. This has been a
problem since at least commit 7ef4a5bd51c606c70661dd303ebd1eeaec0c3834.
I don't know if it is relevant to bring that back though.

Change-Id: Idaa1f30685205e3a62cec2a60a14791d798391a6
Reviewed-on: https://gerrit.libreoffice.org/8748
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>