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.