Allow a BSS entry with all-zeros BSSID to expire

Allow a BSS entry with all-zeros BSSID to expire

wpa_bss_in_use() used to determine that a BSS with BSSID of
00:00:00:00:00:00 is in use in almost every case since either
wpa_s->bssid or wpa_s->pending_bssid was likely to be cleared. This
could result in a corner case of a BSS entry remaining in the BSS table
indefinitely if one was added there with a (likely bogus) address of
00:00:00:00:00:00. Fix this by ignore wpa_s->bssid and
wpa_s->pending_bssid if the BSSID in the BSS table entry is
00:00:00:00:00:00.

In theory, that address is a valid BSSID, but it is unlikely to be used
in any production AP, so the potential expiration of a BSS entry with
that address during a connection attempt would not be a concern
(especially when a new scan would be enough to recover from that).

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>