The hs_metrics_failed_rdv() macro could pass a NULL value for the identity key
when a building circuit would end up in a failure path *before* the "hs_ident"
was able to be set which leading to this assert.
This was introduced in 0.4.8.1-alpha with the addition of rendezvous circuit
failure metrics for the MetricsPort.
This fixes TROVE-2023-006 for which its severity is considered high.
Signed-off-by: David Goulet <dgoulet@torproject.org>
