cfg80211: fix a crash in nl80211_send_station

mac80211 leaves sinfo->assoc_req_ies uninitialized, causing a random
pointer memory access in nl80211_send_station.
Instead of checking if the pointer is null, use sinfo->filled, like
the rest of the fields.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>