Fix the following crash:

  $ qemu-system-x86_64 -object can-host-socketcan,id=obj0
  Segmentation fault (core dumped)

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Message-Id: <20201008202713.1416823-3-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>