Avoid segfault when system has no SMBIOS entry point.

Continue without dmi when no SMBIOS or SMBIOS=0x0 in
/sys/firmware/efi/systab.

One cause of this would be in some virtual machines.

https://bugzilla.novell.com/show_bug.cgi?id=829862

Signed-off-by: Julian Wolf <juwolf@suse.de>
Signed-off-by: Thomas Renninger <trenn@suse.de>