bnc#822170: Let's not even try to export invalid chart objects.

If we do, at best, Excel will complain about the document needing
repair.  At worst Excel will skip some of the other valid drawing
objects from being loaded.

(cherry picked from commit 692878e3bb83c0fc104c5cca946c25ccf2d84ab2)

Conflicts:
	sc/source/filter/xcl97/xcl97rec.cxx

Change-Id: If3794d0ae9d8b44b124020bb12b5369dfebc95ae
Reviewed-on: https://gerrit.libreoffice.org/10899
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>