This fixes an assertion failure in relay_send_end_cell_from_edge_() when an
origin circuit and a cpath_layer = NULL were passed.
A service rendezvous circuit could do such a thing when a malformed BEGIN cell
is received but shouldn't in the first place because the service needs to send
an END cell on the circuit for which it can not do without a cpath_layer.
Fixes#22493
Reported-by: Roger Dingledine <arma@torproject.org>
Signed-off-by: David Goulet <dgoulet@torproject.org>