Panic in peer_info.rs seen in tests

[tomaka]

The following line has just panicked for me in a unit test:

substrate/client/network/src/peer_info.rs

Line 139 in 1aef043

&self.0.endpoints[0] // `endpoints` are non-empty by definition

There is indeed a misunderstanding somewhere.
The information about a node are kept even after the node disconnects, and as such endpoints can reasonably be empty.

cc paritytech/polkadot-sdk#555