mac80211_hwsim: Fix NULL pointer dereference

mac80211_hwsim was crashing when receiving tx information from user
space. Crash happens because txi->rate_driver_data[0] is pointing to a
non valid memory address.

This code path is only used by wmediumd and wmediumd doesn't provide
multiple channel support, so we can pass the channel struct
(data2->channel) directly to mac80211_hwsim_monitor_ack function.
Signed-off-by: Javier Lopez <jlopex@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -2011,7 +2011,7 @@ static int hwsim_tx_info_frame_received_nl(struct sk_buff *skb_2,
 	   (hwsim_flags & HWSIM_TX_STAT_ACK)) {
 		if (skb->len >= 16) {
 			hdr = (struct ieee80211_hdr *) skb->data;
-			mac80211_hwsim_monitor_ack(txi->rate_driver_data[0],
+			mac80211_hwsim_monitor_ack(data2->channel,
 						   hdr->addr2);
 		}
 		txi->flags |= IEEE80211_TX_STAT_ACK;