Trigger panic in fuzzer when peer is mid-handshake

Peer::their_node_id is set to Some during the handshake process.
However, df3ab2e 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.

Update the fuzz test to call broadcast_node_announcement mid-handshake
on an inbound channel in order to trigger a panic.