Refactor code to modify circ->n_chan only by circuit_set_circid_chan_helper
Right now code assigns circ->n_chan before actually keeps it in chan-circid map. Previous value of circ->n_chan used as old_chan by circuit_set_n_circid_chan() and circuit_set_circid_chan_helper()
For example:
```
if (old_chan) {
/*
* If we're changing channels or ID and had an old channel and a non
* zero old ID and weren't marked for close (i.e., we should have been
* attached), detach the circuit. ID changes require this because
* circuitmux hashes on (channel_id, circuit_id).
*/
if (old_id != 0 && (old_chan != chan || old_id != id) &&
!(circ->marked_for_close)) {
tor_assert(old_chan->cmux);
circuitmux_detach_circuit(old_chan->cmux, circ);
}
/* we may need to remove it from the conn-circid map */
search.circ_id = old_id;
search.chan = old_chan;
found = HT_REMOVE(chan_circid_map, &chan_circid_map, &search);
if (found) {
tor_free(found);
if (direction == CELL_DIRECTION_OUT) {
/* One fewer circuits use old_chan as n_chan */
--(old_chan->num_n_circuits);
} else {
/* One fewer circuits use old_chan as p_chan */
--(old_chan->num_p_circuits);
}
}
}
```
It's useless to process such circ->n_chan as old_chan
issue