mac80211_hwsim: avoid NULL access

There's a race condition -- started can be set to true
before channel is set due to the way mac80211 callbacks
currently work (->start should probably pass the channel
we would like to have initially). For now simply add a
check to hwsim to avoid dereferencing the NULL channel
pointer.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -418,6 +418,7 @@ static bool mac80211_hwsim_tx_frame(struct ieee80211_hw *hw,
 			continue;
 
 		if (!data2->started || !hwsim_ps_rx_ok(data2, skb) ||
+		    !data->channel || !data2->channel ||
 		    data->channel->center_freq != data2->channel->center_freq ||
 		    !(data->group & data2->group))
 			continue;