staging: rtl8723bs: prevent ->Ssid overflow in rtw_wx_set_scan()

This code has a check to prevent read overflow but it needs another
check to prevent writing beyond the end of the ->Ssid[] array.

Fixes: 554c0a3a ("staging: Add rtl8723bs sdio wifi driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Denis Efremov (Oracle) <efremov@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>