mac80211: Do not try to associate with an empty SSID

mac80211: Do not try to associate with an empty SSID

It looks like some programs (e.g., NM) are setting an empty SSID with
SIOCSIWESSID in some cases. This seems to trigger mac80211 to try to
associate with an invalid configuration (wildcard SSID) which will
result in failing associations (or odd issues, potentially including
kernel panic with some drivers) if the AP were to actually accept this
anyway).

Only start association process if the SSID is actually set. This
speeds up connection with NM in number of cases and avoids sending out
broken association request frames.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>