These assertions must hold, otherwise the following code would
dereference null pointer.

Change-Id: I2b3887aad7116a7ea5176630a015975b8fc087f9
Reviewed-on: https://gerrit.libreoffice.org/22140Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>