drivers/serial: check the remote device before get name

| #0 0x2119bf1 in rpmsg_get_cpuname rptun/rptun.c:1157
| #1 0x24f97bd in uart_rpmsg_device_destroy serial/uart_rpmsg.c:342
| #2 0x2117d56 in rptun_dev_stop rptun/rptun.c:883
| #3 0x21181d7 in rptun_do_ioctl rptun/rptun.c:922
| #4 0x2119721 in rptun_ioctl_foreach rptun/rptun.c:1086

Signed-off-by: chao an <anchao@xiaomi.com>

drivers/serial/uart_rpmsg.c

@@ -339,7 +339,8 @@ static void uart_rpmsg_device_destroy(FAR struct rpmsg_device *rdev,
   FAR struct uart_dev_s *dev = priv_;
   FAR struct uart_rpmsg_priv_s *priv = dev->priv;
 
-  if (strcmp(priv->cpuname, rpmsg_get_cpuname(rdev)) == 0)
+  if (priv->ept.priv != NULL &&
+      strcmp(priv->cpuname, rpmsg_get_cpuname(rdev)) == 0)
     {
       rpmsg_destroy_ept(&priv->ept);
     }