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: stable@vger.kernel.org
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
[rewrapped commit message, small rewording]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>