qdev: Fix bus dependency of DeviceState::hotpluggable getter

Commit 1a37eca1 (qdev: add
"hotpluggable" property to Device) added a property "hotpluggable" to
each device, with its getter accessing parent_bus->allow_hotplug.

Add a NULL check.

Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

hw/core/qdev.c

@@ -735,7 +735,8 @@ static bool device_get_hotpluggable(Object *obj, Error **err)
     DeviceClass *dc = DEVICE_GET_CLASS(obj);
     DeviceState *dev = DEVICE(obj);
 
-    return dc->hotpluggable && dev->parent_bus->allow_hotplug;
+    return dc->hotpluggable && (dev->parent_bus == NULL ||
+                                dev->parent_bus->allow_hotplug);
 }
 
 static void device_initfn(Object *obj)