autoscan: Fix potential freeing of uninitialized pointer

Commit 7c865c68536825d91e64e1545a1f6ed51526ebff introduced a code
path that can skip extra_ie initialization in wpa_supplicant_scan().
This could result in semi-random crashes when using the autoscan
mechanism. Fix this by initializing extra_ie to NULL.

Signed-hostap: Jouni Malinen <j@w1.fi>

wpa_supplicant/scan.c

@@ -432,7 +432,7 @@ static void wpa_supplicant_scan(void *eloop_ctx, void *timeout_ctx)
 	struct wpa_supplicant *wpa_s = eloop_ctx;
 	struct wpa_ssid *ssid;
 	int scan_req = 0, ret;
-	struct wpabuf *extra_ie;
+	struct wpabuf *extra_ie = NULL;
 	struct wpa_driver_scan_params params;
 	struct wpa_driver_scan_params *scan_params;
 	size_t max_ssids;