xmerge: Avoid a possible null pointer exception

Change-Id: Ib6ce07aef1ae27f4e5db376667617432de9f41ef
Reviewed-on: https://gerrit.libreoffice.org/10686
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>