Migration/colo.c: Fix double close bug when occur COLO failover

In migration_incoming_state_destroy(void) will check the mis->to_src_file
to double close the mis->to_src_file when occur COLO failover.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190303145021.2962-2-chen.zhang@intel.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/colo.c

@@ -872,6 +872,7 @@ out:
     /* Must be called after failover BH is completed */
     if (mis->to_src_file) {
         qemu_fclose(mis->to_src_file);
+        mis->to_src_file = NULL;
     }
     migration_incoming_disable_colo();