pPostIt is 0 when we have a large document, and the layout is not yet
ready for a later part of the document, having a comment. Just don't try
to delete such comments, better than a crash.

(cherry picked from commit 9a764145)

Change-Id: I5fd5c908bc7b1feba926214863e84391b9037484
Reviewed-on: https://gerrit.libreoffice.org/3757Reviewed-by: Bosdonnat Cedric <cedric.bosdonnat@free.fr>
Tested-by: Bosdonnat Cedric <cedric.bosdonnat@free.fr>