Fix (and DRY) the conditionals before calling `peer_disconnected`

Fix (and DRY) the conditionals before calling `peer_disconnected`

If we have a peer that sends a non-`Init` first message, we'll call
`peer_disconnected` without ever having called `peer_connected`
(which has to wait until we have an `Init` message). This is a
violation of our API guarantees, though should generally not be an
issue.

Because this bug was repeated in a few places, we also take this
opportunity to DRY up the logic which checks the peer state before
calling `peer_disconnected`.

Found by the new `ChannelManager` assertions and the
`full_stack_target` fuzzer.