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>

hw/ppc/spapr.c

@@ -2725,6 +2725,7 @@ static void spapr_core_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
     CPUCore *cc = CPU_CORE(dev);
     CPUArchId *core_slot = spapr_find_cpu_slot(ms, cc->core_id, NULL);
 
+    assert(core_slot);
     core_slot->cpu = NULL;
     object_unparent(OBJECT(dev));
 }