assert(meshlink_channel_send(mesh_a, channel, buf, 29) == 0);
assert(meshlink_channel_send(mesh_a, channel, buf, 513) == -1);
assert(meshlink_channel_send(mesh_a, channel, buf, 29) == 0);
assert(meshlink_channel_send(mesh_a, channel, buf, 513) == -1);