vl: Assert drive_new() does not fail in default_drive()

If creating (empty) default drives fails, it's a bug.  Therefore,
assert() is more appropriate than exit(1).

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20181017082702.5581-33-armbru@redhat.com>
1 file changed