wifi: at76c50x-usb: avoid length underflow in at76_guess_freq()

wifi: at76c50x-usb: avoid length underflow in at76_guess_freq()

at76_guess_freq() checks only that the received frame is at least a bare
802.11 header (24 bytes) before subtracting the fixed management-body
offset:

	len -= el_off;

For both beacon and probe response frames, el_off is 36. If the frame is
shorter than el_off, subtracting it causes the calculated IE length to
wrap. The length is eventually passed to cfg80211_find_elem_match() as a
very large unsigned value, so the element walk runs beyond the RX skb.

This path is reached from at76_rx_tasklet() while scanning. If the device
delivers a truncated beacon or probe response, the oversized IE length
causes an out-of-bounds read during scanning.

Skip the IE lookup if the frame does not reach the variable elements,
before subtracting el_off.

Fixes: 1264b951463a ("at76c50x-usb: add driver")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Huihui Huang <hhhuang@smu.edu.sg>
Link: https://patch.msgid.link/20260715140815.1242033-1-hhhuang@smu.edu.sg
Signed-off-by: Johannes Berg <johannes.berg@intel.com>