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: Tim Harvey <harvey.tim@gmail.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>