station: Make sure roam_scan_id is always canceled

Under very rare circumstances the roaming scan triggered might not be
canceled properly.  This is because we issue the roam scan recursively
from within a scan callback and re-use the id of the scan for the
subsequent request.  The destroy callback is invoked right after the
callback and resets the id.  This leads to the scan not being canceled
properly in roam_state_clear().

src/netdev.c:netdev_mlme_notify() MLME notification Notify CQM(64)
src/station.c:station_roam_trigger_cb() 37
src/station.c:station_roam_scan() ifindex: 37
src/station.c:station_roam_trigger_cb() Using cached neighbor report for roam
...
src/scan.c:get_scan_done() get_scan_done
src/station.c:station_roam_failed() 37
src/station.c:station_roam_scan() ifindex: 37
src/scan.c:scan_request_triggered() Active scan triggered for wdev 22
^CTerminate
src/netdev.c:netdev_free() Freeing netdev wlan0[37]
src/device.c:device_free()
src/station.c:station_free()
...
Removing scan context for wdev 22
src/scan.c:scan_context_free() sc: 0x4a362a0
src/wiphy.c:wiphy_radio_work_done() Work item 14 done
==19542== Invalid write of size 4
==19542==    at 0x411500: station_roam_scan_destroy (station.c:2010)
==19542==    by 0x420B5B: scan_request_free (scan.c:156)
==19542==    by 0x410BAC: destroy_work (wiphy.c:294)
==19542==    by 0x410BAC: wiphy_radio_work_done (wiphy.c:1613)
==19542==    by 0x46C66E: l_queue_clear (queue.c:107)
==19542==    by 0x46C6B8: l_queue_destroy (queue.c:82)
==19542==    by 0x420BAE: scan_context_free (scan.c:205)
==19542==    by 0x424135: scan_wdev_remove (scan.c:2272)
==19542==    by 0x408754: netdev_free (netdev.c:847)
==19542==    by 0x40E18C: netdev_shutdown (netdev.c:5773)
==19542==    by 0x404756: iwd_shutdown (main.c:78)
==19542==    by 0x404756: iwd_shutdown (main.c:65)
==19542==    by 0x470E21: handle_callback (signal.c:78)
==19542==    by 0x470E21: signalfd_read_cb (signal.c:104)
==19542==    by 0x47166B: io_callback (io.c:120)
==19542==  Address 0x4d81f98 is 200 bytes inside a block of size 288 free'd
==19542==    at 0x48399CB: free (vg_replace_malloc.c:538)
==19542==    by 0x47F3E5: interface_instance_free (dbus-service.c:510)
==19542==    by 0x481DEA: _dbus_object_tree_remove_interface (dbus-service.c:1694)
==19542==    by 0x481F1C: _dbus_object_tree_object_destroy (dbus-service.c:795)
==19542==    by 0x40894F: netdev_free (netdev.c:844)
==19542==    by 0x40E18C: netdev_shutdown (netdev.c:5773)
==19542==    by 0x404756: iwd_shutdown (main.c:78)
==19542==    by 0x404756: iwd_shutdown (main.c:65)
==19542==    by 0x470E21: handle_callback (signal.c:78)
==19542==    by 0x470E21: signalfd_read_cb (signal.c:104)
==19542==    by 0x47166B: io_callback (io.c:120)
==19542==    by 0x47088C: l_main_iterate (main.c:478)
==19542==    by 0x47095B: l_main_run (main.c:525)
==19542==    by 0x47095B: l_main_run (main.c:507)
==19542==    by 0x470B6B: l_main_run_with_signal (main.c:647)
==19542==  Block was alloc'd at
==19542==    at 0x483879F: malloc (vg_replace_malloc.c:307)
==19542==    by 0x46AB2D: l_malloc (util.c:62)
==19542==    by 0x416599: station_create (station.c:3448)
==19542==    by 0x406D55: netdev_newlink_notify (netdev.c:5324)
==19542==    by 0x46D4BC: l_hashmap_foreach (hashmap.c:612)
==19542==    by 0x472F46: process_broadcast (netlink.c:158)
==19542==    by 0x472F46: can_read_data (netlink.c:279)
==19542==    by 0x47166B: io_callback (io.c:120)
==19542==    by 0x47088C: l_main_iterate (main.c:478)
==19542==    by 0x47095B: l_main_run (main.c:525)
==19542==    by 0x47095B: l_main_run (main.c:507)
==19542==    by 0x470B6B: l_main_run_with_signal (main.c:647)
==19542==    by 0x403EDB: main (main.c:490)
==19542==

src/station.c

@@ -1605,9 +1605,18 @@ static void station_roam_failed(struct station *station)
 	 * If we tried a limited scan, failed and the signal is still low,
 	 * repeat with a full scan right away
 	 */
-	if (station->signal_low && !station->roam_scan_full &&
+	if (station->signal_low && !station->roam_scan_full) {
-					!station_roam_scan(station, NULL))
+		/*
+		 * Since we're re-using roam_scan_id, explicitly cancel
+		 * the scan here, so that the destroy callback is not called
+		 * after the return of this function
+		 */
+		scan_cancel(netdev_get_wdev_id(station->netdev),
+						station->roam_scan_id);
+
+		if (!station_roam_scan(station, NULL))
 			return;
+	}
 
 delayed_retry:
 	/*