spapr: ensure core_slot isn't NULL in spapr_core_unplug()

If we go that far on the path of hot-removing a core and we find out that
the core-id is invalid, then we have a serious bug.

Let's make it explicit with an assert() instead of dereferencing a NULL
pointer.

This fixes Coverity issue CID 1375404.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>