wifi: libipw: fix key index receive bound checks

In the Linux kernel, the following vulnerability has been resolved: wifi: libipw: fix key index receive bound checks libipw_rx() reads skb->data[hdrlen + 3] to extract the WEP key index in both the software-decrypt key selection path and the hardware-decrypted IV/ICV strip path. In both places the existing guard only checks skb->len >= hdrlen + 3, which proves bytes up to hdrlen + 2 but not the byte at hdrlen + 3. Require hdrlen + 4 bytes before reading that item in both paths. This is a local source-boundary check only; it does not change the key index semantics.