qapi_event_send_mem_unplug_error() deals with @device being NULL by
replacing it with an empty string ("") when emitting the event. Aside
from the fact that this behavior (qapi visitor mapping NULL pointer to
"") can be patched/changed someday, there's also the lack of utility
that the event brings to listeners, e.g. "a memory unplug error happened
somewhere".

In theory we should just avoit emitting this event at all if dev->id is
NULL, but this would be an incompatible change to existing guests.
Instead, let's make the forementioned behavior explicit: if dev->id is
NULL, pass an empty string to qapi_event_send_mem_unplug_error().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 hw/acpi/memory_hotplug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/acpi/memory_hotplug.c b/hw/acpi/memory_hotplug.c
index af37889423..6a71de408b 100644
--- a/hw/acpi/memory_hotplug.c
+++ b/hw/acpi/memory_hotplug.c
@@ -178,7 +178,7 @@ static void acpi_memory_hotplug_write(void *opaque, hwaddr addr, uint64_t data,
             hotplug_handler_unplug(hotplug_ctrl, dev, &local_err);
             if (local_err) {
                 trace_mhp_acpi_pc_dimm_delete_failed(mem_st->selector);
-                qapi_event_send_mem_unplug_error(dev->id,
+                qapi_event_send_mem_unplug_error(dev->id ? : "",
                                                  error_get_pretty(local_err));
                 error_free(local_err);
                 break;
-- 
2.31.1