Avoid panic when 0conf channel's ann. sigs race on-chain confirmation

Avoid panic when 0conf channel's ann. sigs race on-chain confirmation

A channel's `short_channel_id` is currently only set when the funding
transaction is confirmed via `transactions_confirmed`, which might be
well after the channel initally becomes usable, e.g., in the 0conf case.

Previously we would panic due to a reachable `unwrap` when receiving a
counterparty's `announcement_signatures` message for a 0conf channel
pending confirmation on-chain.

Here we fix this bug by avoiding unsafe `unwrap`s and just erroring out
and ignoring the announcement_signatures message if the `short_channel_id`
hasn't been set yet.