Skip networks without known SSID when selecting the BSS

Previously, APs that were hiding SSID (zero-length SSID IE in
Beacon frames) could have been selected when wildcard SSID matching
was used. This would result in failed association attempt since
the client does not know the correct SSID. This can slow down WPS
which is often using wildcard SSID matching.

Ignore BSSes without known SSID in the scan results when selecting
which BSS to use.

wpa_supplicant/events.c

@@ -427,6 +427,11 @@ wpa_supplicant_select_bss_wpa(struct wpa_supplicant *wpa_s,
 			continue;
 		}
 
+		if (ssid_len == 0) {
+			wpa_printf(MSG_DEBUG, "   skip - SSID not known");
+			continue;
+		}
+
 		if (wpa_ie_len == 0 && rsn_ie_len == 0) {
 			wpa_printf(MSG_DEBUG, "   skip - no WPA/RSN IE");
 			continue;
@@ -517,6 +522,11 @@ wpa_supplicant_select_bss_non_wpa(struct wpa_supplicant *wpa_s,
 			continue;
 		}
 
+		if (ssid_len == 0) {
+			wpa_printf(MSG_DEBUG, "   skip - SSID not known");
+			continue;
+		}
+
 		for (ssid = group; ssid; ssid = ssid->pnext) {
 			int check_ssid = ssid->ssid_len != 0;