net/semantic/parser: fix compile warning found by sparse

Reference:
https://linux.die.net/man/1/sparse

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

net/netdev/netdev_ioctl.c

@@ -1423,7 +1423,7 @@ static int netdev_rt_ioctl(FAR struct socket *psock, int cmd,
         {
           /* The target address and the netmask are required values */
 
-          if (rtentry == 0)
+          if (rtentry == NULL)
             {
               return -EINVAL;
             }