Check miner tx has a miner input when deserializing.

[Boog900]

No description provided.

[kayabaNerve]

This doesn't resolve the panic noted in fn number as it's still possible to manually construct a block without a miner TX. While I'm fine merging this, do you want to fix that in this PR, in a new PR, or shall I follow up there? (any is fine with me)

[Boog900]

I didn't want to change the API of the number function and IMO if someone is creating invalid blocks manually panicking is probably ok (should be documented though). I had a look and I think it would probably be best for you to decide how to handle invalid miner txs in number as its used in places where i'm unsure what should be done.