mtp: fix initialization to avoid segfault on close

When an Android device is plugged with MTP disabled, the tracks may not
be retrieved.

In that branch of code, pp_items was never initialized, so the behavior
was undefined in CloseDevice() (on device unplug), often causing a
segfault.

Initialize it to NULL to fix the problem.
Signed-off-by: Romain Vimont <rom@rom1v.com>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/services_discovery/mtp.c

@@ -198,6 +198,7 @@ static int AddDevice( services_discovery_t *p_sd,
                             CountTracks, p_sd ) ) == NULL )
         {
             msg_Warn( p_sd, "No tracks on the device" );
+            p_sd->p_sys->pp_items = NULL;
         }
         else
         {