mac80211: Fix NULL-pointer deference on ibss merge when not ready

dev_open will eventually call ieee80211_ibss_join which sets up the
skb used for beacons/probe-responses however it is possible to
receive beacons that attempt to merge before this occurs causing
a null pointer dereference.  Check ssid_len as that is the last
thing set in ieee80211_ibss_join.

This occurs quite easily in the presence of adhoc nodes with hidden SSID's

revised previous patch to check further up based on irc feedback
Signed-off-by: NTim Harvey <harvey.tim@gmail.com>
Reviewed-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/ibss.c

@@ -780,6 +780,9 @@ void ieee80211_ibss_rx_queued_mgmt(struct ieee80211_sub_if_data *sdata,
 
 	mutex_lock(&sdata->u.ibss.mtx);
 
+	if (!sdata->u.ibss.ssid_len)
+		goto mgmt_out; /* not ready to merge yet */
+
 	switch (fc & IEEE80211_FCTL_STYPE) {
 	case IEEE80211_STYPE_PROBE_REQ:
 		ieee80211_rx_mgmt_probe_req(sdata, mgmt, skb->len);
@@ -797,6 +800,7 @@ void ieee80211_ibss_rx_queued_mgmt(struct ieee80211_sub_if_data *sdata,
 		break;
 	}
 
+ mgmt_out:
 	mutex_unlock(&sdata->u.ibss.mtx);
 }