Fix panic when peer is mid-handshake

Fix panic when peer is mid-handshake

Peer::their_node_id is set to Some during the handshake process.
However, df3ab2ee2753e7f9ec02ddf1c8a51db77c50e35d accesses the field
unconditionally, causing a panic. This may be triggered if a gossip
message is received mid-handshake from another peer or if the user calls
broadcast_node_announcement during this time. The latter tends to be
executed on a timer.

Ensure that Peer::their_node_id is only accessed once the handshake is
complete.