Detail: Avoid dereferencing some NULL pointers.

Detail: Avoid dereferencing some NULL pointers.

dm devices which only have a single underlying md device
will respond to md ioctls  as though they were that md device.
This can confuse mdadm and lead it to violating its segments.

So add tests for NULL where appropriate.  You might not get exactly
the right answer when you "mdadm -D" a dm device, but at least it won't
crash now.

Reported-by: Willy Weisz <Willy.Weisz@univie.ac.at>
Resolves: https://bugzilla.novell.com/show_bug.cgi?id=887821
Signed-off-by: NeilBrown <neilb@suse.de>