mac80211: fix SSID copy on IBSS JOIN

The 'ssid' field of the cfg80211_ibss_params is a u8 pointer and
its length is likely to be less than IEEE80211_MAX_SSID_LEN most
of the time.

This patch fixes the ssid copy in ieee80211_ibss_join() by using
the SSID length to prevent it from reading beyond the string.

Cc: [e-mail]
Signed-off-by: Antonio Quartulli <[e-mail]>
[rewrapped commit message, small rewording]
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/ibss.c

···
 	sdata->u.ibss.state = IEEE80211_IBSS_MLME_SEARCH;
 	sdata->u.ibss.ibss_join_req = jiffies;
 
-	memcpy(sdata->u.ibss.ssid, params->ssid, IEEE80211_MAX_SSID_LEN);
+	memcpy(sdata->u.ibss.ssid, params->ssid, params->ssid_len);
 	sdata->u.ibss.ssid_len = params->ssid_len;
 
 	mutex_unlock(&sdata->u.ibss.mtx);