hs-v3: Remove a possible BUG() condition
When receiving an introduction NACK, the client either decides to close or re-extend the circuit to another intro point.
In order to do this, the service descriptor needs to exists but it is possible that it gets removed from the cache between the establishement of the introduction circuit and the reception of the (N)ACK.
For that reason, the BUG(desc == NULL) is removed because it is a possible normal use case. Tor recovers gracefully already.
Fixes #34087 (closed)
Signed-off-by: David Goulet firstname.lastname@example.org