common/bolt11: validate public keys in routing hints

[erickcestari]

This PR adds validation of public keys in BOLT11 routing hints to prevent processing of malformed public keys. Without this validation, invalid public keys could be accepted.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
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• Documentation has been reviewed and updated as needed.
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[rustyrussell]

Ack e93fe72

[rustyrussell]

Thanks! This is great spotting. Would not have happened if we'd use a struct pubkey instead of struct node_id for that type, which I think we should seriously consider, though that's a larger change.