CLEANUP: connection: Indicate unreachability to the compiler in conn_recv_proxy

Transform the unreachability comment into a call to `my_unreachable()` to allow
the compiler from benefitting from it.

see d1b15b6e9 ("MINOR: proxy_protocol: Ingest PP2_TYPE_UNIQUE_ID on incoming connections")
see 615f81eb5 ("MINOR: connection: Use a `struct ist` to store proxy_authority")
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