`psbt_contribs_changed` modifies its inputs and loses the final witness field about lightning HOT 3 CLOSED

I discussed this with @ksedgwic on Matrix. Looks like the error is gone if we remove this lightning-signer@19027e5 code

I do not know what is the reason for this code, but I am guessing that a plugin can change a PSBT and here we are making sure that it is sane. However, this looks like a bug if we break VLS. There should be another way.

cc @niftynei Can you help me understand the reason for this code? I get the logic right?

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Found the specific problem (it's near the sort, but the sort is fine):

• psbt_contribs_changed -> input_identical -> linearize_input which calls:

	wally_psbt_input_set_final_witness(&psbt->inputs[0], NULL);

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Ah, do you have the pointer for the spec? we should add it inside the code if it is spec related

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