[mutter] backends/x11: Fix use after free on device removal

[Robert Mader <rmader src gnome org>]

commit 81de2c2913a73fe2487c880112cf3815b802a5ed
Author: Sebastian Keller <skeller gnome org>
Date:   Sat Feb 1 02:28:12 2020 +0100

    backends/x11: Fix use after free on device removal
    
    The devices_by_id hash table is responsible for managing the reference
    to the devices. In remove_device however, for non-core devices there are
    additional calls to dispose/unref, after the last reference has
    already been dropped by the hash table.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/1032

 src/backends/x11/meta-seat-x11.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)
---
diff --git a/src/backends/x11/meta-seat-x11.c b/src/backends/x11/meta-seat-x11.c
index 9a43a2762..4c2395351 100644
--- a/src/backends/x11/meta-seat-x11.c
+++ b/src/backends/x11/meta-seat-x11.c
@@ -698,12 +698,11 @@ remove_device (MetaSeatX11 *seat_x11,
         {
           seat_x11->devices = g_list_remove (seat_x11->devices, device);
           g_signal_emit_by_name (seat_x11, "device-removed", device);
-          g_hash_table_remove (seat_x11->devices_by_id,
-                               GINT_TO_POINTER (device_id));
         }
 
       g_object_run_dispose (G_OBJECT (device));
-      g_object_unref (device);
+      g_hash_table_remove (seat_x11->devices_by_id,
+                           GINT_TO_POINTER (device_id));
     }
 }