Fix route->channels double-free corruption.

The code assumes that route->channels is NULL when route->channels_len
is 0, such that free(route->channels) will work.

Think about this scenario:
  update(r, some channels)  # route->channels = malloc(…)
  update(r, no channel)  # free(route->channels)
  update(r, no channel)  # free(route->channels)

Thanks to Dave Taht for pointing the issue.

route.c

@@ -918,6 +918,7 @@ update_route(const unsigned char *id,
 
         if(channels_len == 0) {
             free(route->channels);
+            route->channels = NULL;
             route->channels_len = 0;
         } else {
             if(channels_len != route->channels_len) {