staging: rtl8188eu: Fix null pointer dereference on free_netdev call

An unregister_netdev call checks if pnetdev is null, hence a later
call to free_netdev can potentially be passing a null pointer, causing
a null pointer dereference. Avoid this by adding a null pointer check
on pnetdev before calling free_netdev.

Fixes: 1665c8fdff ("staging: rtl8188eu: use netdev routines for private data")
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210324152135.254152-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/os_dep/usb_intf.c

@@ -446,7 +446,8 @@ static void rtw_usb_if1_deinit(struct adapter *if1)
 	pr_debug("+r871xu_dev_remove, hw_init_completed=%d\n",
 		 if1->hw_init_completed);
 	rtw_free_drv_sw(if1);
-	free_netdev(pnetdev);
+	if (pnetdev)
+		free_netdev(pnetdev);
 }
 
 static int rtw_drv_init(struct usb_interface *pusb_intf, const struct usb_device_id *pdid)