bark: validate HTLC VTXO backing tree before disclosing preimage

Duncan Deanrequested to merge
duncan/vul-125-preimage-disclosed-for-an-unsafe-htlc-backing-tree into master

We reuse validate_vtxo_tree_params to validate the tree backing an HTLC VTXO in lightning receive before claiming with preimage.

This is only exploitable by the server and not by a third party.

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