Related: rhbz#1076264 intermittent a11y crash in calc

http://retrace.fedoraproject.org/faf/reports/338291/

this crash refuses to be captured alive. But seeing as
we sometimes check mpViewShell against NULL and sometimes check
GetViewData against NULL its worth making those checks everytime
for consistency

see also...

rhbz#923993 rhbz#896711 rhbz#602273 rhbz#577595 lp#811057 lp#760665

(cherry picked from commit fae1e19af05e664364c55d4f686aaab860fd538d)

Conflicts:
	sc/source/ui/Accessibility/AccessibleDocument.cxx

Change-Id: I5553848018f500b63ade60bcfc756a736b27c64f
Reviewed-on: https://gerrit.libreoffice.org/8604
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>