mac80211_hwsim: check txrate for NULL

If the rate control algorithm messed up then the txrate pointer
here could be NULL - WARN and drop the packet from monitoring.

Signed-off-by: Amit Khatri <amit.khatri@samsung.com>
Signed-off-by: Rahul Jain <rahul.jain@samsung.com>
[rewrite commit message, add warning]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -814,6 +814,9 @@ static void mac80211_hwsim_monitor_rx(struct ieee80211_hw *hw,
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(tx_skb);
 	struct ieee80211_rate *txrate = ieee80211_get_tx_rate(hw, info);
 
+	if (WARN_ON(!txrate))
+		return;
+
 	if (!netif_running(hwsim_mon))
 		return;