Do not proceed with association if get_bssid() returns failure

Do not proceed with association if get_bssid() returns failure

This is the normal flow for association:

wpa_supplicant <--(EVENT_ASSOC event )--  device driver
wpa_supplicant  --( get_bssid()      )--> device driver
wpa_supplicant <--( return BSSID     )--  device driver

However, a device driver could return EINVAL for get_bssid() because it
recognizes it has already been disconnected. When the wpa_supplicant
received EINVAL, the bssid field could be used uninitialized in the
following flow:

wpa_supplicant <--(EVENT_ASSOC event )--  device driver
                                          device driver (receive deauth)
wpa_supplicant  --( get_bssid()      )--> device driver
wpa_supplicant <--( return EINVAL    )--  device driver

Prevent this by requiring the get_bssid() call to succeed when
processing association events.