system: make qemu_del_vm_change_state_handler accept NULL

For convenience.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

system/runstate.c

@@ -349,6 +349,9 @@ VMChangeStateEntry *qemu_add_vm_change_state_handler(VMChangeStateHandler *cb,
 
 void qemu_del_vm_change_state_handler(VMChangeStateEntry *e)
 {
+    if (!e) {
+        return;
+    }
     QTAILQ_REMOVE(&vm_change_state_head, e, entries);
     g_free(e);
 }