ath6kl: prevent potential array overflow in ath6kl_add_new_sta()

The value for "aid" comes from skb->data so Smatch marks it as
untrusted.  If it's invalid then it can result in an out of bounds array
access in ath6kl_add_new_sta().

Fixes: 572e27c0 ("ath6kl: Fix AP mode connect event parsing and TIM updates")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200813141315.GB457408@mwanda

drivers/net/wireless/ath/ath6kl/main.c

@@ -430,6 +430,9 @@ void ath6kl_connect_ap_mode_sta(struct ath6kl_vif *vif, u16 aid, u8 *mac_addr,
 
 	ath6kl_dbg(ATH6KL_DBG_TRC, "new station %pM aid=%d\n", mac_addr, aid);
 
+	if (aid < 1 || aid > AP_MAX_NUM_STA)
+		return;
+
 	if (assoc_req_len > sizeof(struct ieee80211_hdr_3addr)) {
 		struct ieee80211_mgmt *mgmt =
 			(struct ieee80211_mgmt *) assoc_info;