memory: make memory_listener_unregister idempotent

Make it easy to unregister a MemoryListener without tracking whether it
had been registered before.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

memory.c

@@ -2371,8 +2371,13 @@ void memory_listener_register(MemoryListener *listener, AddressSpace *as)
 
 void memory_listener_unregister(MemoryListener *listener)
 {
+    if (!listener->address_space) {
+        return;
+    }
+
     QTAILQ_REMOVE(&memory_listeners, listener, link);
     QTAILQ_REMOVE(&listener->address_space->listeners, listener, link_as);
+    listener->address_space = NULL;
 }
 
 void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)