conflux_validate_legs(): Bug: Number of legs is above maximum of 2 allowed: 3#012 (on Tor 0.4.8.3-rc-dev )

A cypherpunks user reported some logspam due to this BUG check as a random comment in #40834 (comment 2935264). Initially, this looks like a rogue Tor client hitting this check. This has never happened in a Shadow run.

This is harmless, but noisy. It is also at warn and not protocol warn.

We could remove the BUG() and demote to ProtocolWarn, but should have a look through the client codepaths for cases that could somehow spawn 3 legs instead of 2, just in case.