check_drive: If the partition isn't a Vinum drive, release it again
and return NULL.

vinum_scandisk: Don't handle NULL device pointers.

	        Only look at compatibility partition for i386.  This
	        is a kludge which should go away once I have adequate
	        documentation for the New World Order.

Together, these fixes remove occasional error messages about
non-existent drives. They may also fix a number of problems that have
been reported without a PR.

PRs: None