mdadm/ddf: Abort when raid disk is smaller in getinfo_super_ddf

mdadm/ddf: Abort when raid disk is smaller in getinfo_super_ddf

The metadata is corrupted when the raid_disk<0. So abort directly.
This also can avoid a building error:
super-ddf.c:1988:58: error: array subscript -1 is below array bounds of ‘struct phys_disk_entry[0]’

Suggested-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Ackedy-by: Xiao Ni <xni@redhat.com>
Acked-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>