Fix TRACK_STA_LIST before BSS enabled

This hostapd control interface command could hit a NULL pointer
dereference if issued before the BSS was enabled.

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

hostapd/ctrl_iface.c

@@ -2040,6 +2040,9 @@ static int hostapd_ctrl_iface_track_sta_list(struct hostapd_data *hapd,
 	struct hostapd_sta_info *info;
 	struct os_reltime now;
 
+	if (!iface->num_sta_seen)
+		return 0;
+
 	sta_track_expire(iface, 0);
 
 	pos = buf;