Reject transactions that spend a treasury UTXO without creating a new one

[1440000bytes]

Per BIP300 a transaction that spends a treasury UTXO as an input and creates no new treasury UTXO as an output is invalid.

https://github.com/LayerTwo-Labs/bip300_bip301_specifications/blob/master/bip300.md#validation-rules-specification

handle_m5_m6 builds ctip_spends (treasury inputs) but only consults it inside the loop over new_ctips (treasury outputs). A tx that spends a treasury UTXO and creates no new treasury output never enters that loop, so it returns Ok(None) and is ignored. OP_DRIVECHAIN is anyone-can-spend, so anyone could drain a sidechain's treasury this way.

This adds the missing check. Every spent treasury must have a corresponding new treasury output, otherwise the tx is invalid. Block-level rejection of the invalid tx is handled by #365 (which propagates non-fatal tx errors instead of
skipping them).

[Ash-L2L]

Thanks! LGTM