drivers/serial/pty: fix memory leak when pty_destroy

1. ptmx_open -> pty_register2 -> master register i_crefs 1
                              -> salve  register i_crefs 1
2. ptmx_open -> master unregister i_crefs 0
3. master open i_crefs 1
   salve  open i_crefs 2

4. in pty_close() master close with i_crefs 1
                  salve  close with i_crefs 2

So, if correct the i_crefs judgement in pty_close()

Signed-off-by: ligd <liguiding1@xiaomi.com>
Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

Author: GUIDINGLI

drivers/serial/pty.c

@@ -340,7 +340,8 @@ static int pty_close(FAR struct file *filep)
 
   /* Check if the decremented inode reference count would go to zero */
 
-  if (atomic_read(&inode->i_crefs) == 1)
+  if ((!dev->pd_master && atomic_load(&inode->i_crefs) == 2) ||
+       (dev->pd_master && atomic_load(&inode->i_crefs) == 1))
     {
       /* Did the (single) master just close its reference? */