hostapd: Fix segfault after ACS when flushing STAs

When hostapd receives an auth frame during ACS the transmission of
the according auth response will always fail:

ACS: Automatic channel selection started, this may take a bit
[..]
send_auth_reply: send: Resource temporarily unavailable
[..]

However, a station info entry was created. Once ACS is finished
it will flush all stations even though hapd was not yet fully
initialized. This results in a segfault when trying to access
hapd->radius:

0  0x0042c1c0 in radius_client_flush_auth ()
1  0x00416a94 in ap_free_sta ()
2  0x00416cc0 in hostapd_free_stas ()
3  0x0040bce8 in hostapd_flush_old_stations ()
4  0x0040c790 in hostapd_setup_interface_complete ()
5  0x0046347c in acs_scan_complete ()
6  0x0040f834 in hostapd_wpa_event ()
7  0x0043af08 in send_scan_event.part.46 ()
8  0x00443a64 in send_scan_event ()
9  0x00443c24 in do_process_drv_event ()
10 0x004449e8 in process_global_event ()
11 0x7767d7d0 in ?? ()

Fix this by not presuming anything about the initialization state of
hapd and checking ->radius before accessing.

Signed-off-hostapd: Helmut Schaa <helmut.schaa@googlemail.com>

src/ap/sta_info.c

@@ -232,6 +232,7 @@ void ap_free_sta(struct hostapd_data *hapd, struct sta_info *sta)
 	wpa_auth_sta_deinit(sta->wpa_sm);
 	rsn_preauth_free_station(hapd, sta);
 #ifndef CONFIG_NO_RADIUS
+	if (hapd->radius)
 		radius_client_flush_auth(hapd->radius, sta->addr);
 #endif /* CONFIG_NO_RADIUS */