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: 1665c8fdffbb ("staging: rtl8188eu: use netdev routines for private data")
Reviewed-by: Martin Kaiser <[e-mail]>
Signed-off-by: Colin Ian King <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/rtl8188eu/os_dep/usb_intf.c

···
 	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)