cfg80211: fix potential connection work crash

If wpa_supplicant and iw/iwconfig are used together, very
rarely the system crashes. It seems to be related to the
connection parameters not being set up, but it's not all
clear to me how this happens. In any case, checking that
the conn pointer exists here is probably a good idea.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>