Found by the UBSAN tinderbox.

Change-Id: I0c23f316713bb6c8abc331a1a2d131a068962812
Reviewed-on: https://gerrit.libreoffice.org/40752Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>