wifi: mac80211: validate deauth frame length before reason access

wifi: mac80211: validate deauth frame length before reason access

ieee80211_rx_mgmt_deauth() reads the deauth reason code before checking
that the fixed field is actually present in the received frame.

Validate the deauth frame length first and only then read the reason
code.

Assisted-by: Codex:gpt-5.5
Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Zhao Li <enderaoelyther@gmail.com>
Link: https://patch.msgid.link/20260612185042.66260-6-enderaoelyther@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>