Use ssh with proxy_sock has some problem

When I use ssh with proxy_sock and proxy_sock is `unix domain socket`, getsockname will return '', and sockname[0] will cause an Exception

https://github.com/Gallopsled/pwntools/blob/7e03bfd5a659afeaa165094b39e0ecec90c547c4/pwnlib/tubes/ssh.py#L459-L461

Start by reading the proxy socket handling around pwnlib/tubes/ssh.py lines 459-461, focusing on the behavior of getsockname for a Unix domain socket. Reproduce the exception with a Unix socket, then ensure this case is handled without indexing an empty result and verify the SSH proxy behavior.

Written by the indexing model from the issue text.