fdo#86795: xmloff: ODF import: do not import checkbox fieldmark ...

... if it has start/end, because Writer will become grumpy and crash.

(cherry picked from commit 25fd11e78279aef5a6b7656347758e5c67a9c45a)

Conflicts:
	xmloff/source/text/XMLTextMarkImportContext.cxx

Change-Id: I6024051249eeac6ed9e43856fa77db969287f888
Reviewed-on: https://gerrit.libreoffice.org/13319
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>