backup-locker.c: Kill xprop during unlock.

In the old screensaver, xprop monitored the screensaver window.
When the screensaver was unlocked, that window would be destroyed,
and xprop would exit on its own.

Now, it monitors Cinnamon's stage window, which never gets destroyed,
so we need to explicitly kill xprop when unlocking to prevent it
getting orphaned when the backup-locker exits.

Author: mtwebster

src/screensaver/backup-locker/backup-locker.c

@@ -716,14 +716,17 @@ window_monitor_thread (GTask        *task,
             }
             else
             {
-                g_debug ("xprop cancelled");
+                g_debug ("xprop cancelled, killing subprocess");
+                g_subprocess_send_signal (xprop_proc, SIGTERM);
             }
         }
         else
         {
             gint exit_status = g_subprocess_get_exit_status (xprop_proc);
             g_debug ("xprop exited (status=%d)", exit_status);
         }
+
+        g_object_unref (xprop_proc);
     }
     g_clear_error (&error);
     g_task_return_boolean (task, TRUE);