As pointed out by sberg, either the issue is a programming error that
needs an assert, or the issue is unusual and should just generated a
warning, but never both.

Change-Id: I07ae1e99597cd6c342791aa268b2240db987ae91
Reviewed-on: https://gerrit.libreoffice.org/39514Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>