Fix #424 by iterating original_input on match (#428)

The payjoin psbt construction code reintroduces original psbt inputs that
the receiver may have lost. It does this by iterating through each
proposed input and original input.

This fix only iterates to the next original_input once a proposed input
that matches is found, otherwise inputs could be skipped and signing
could fail. The outer loop already iterates through proposed inputs.
Inputs in the proposal are in the same order as the original as per spec,
and just insert receiver inputs so this iteration will always find matches.

src/bitcoin/payment.rs

@@ -175,8 +175,8 @@ fn add_back_original_input(original_psbt: &Psbt, payjoin_psbt: Psbt) -> Psbt {
             if proposed_txin.previous_output == original_txin.previous_output {
                 proposed_psbtin.witness_utxo = original_psbtin.witness_utxo.clone();
                 proposed_psbtin.non_witness_utxo = original_psbtin.non_witness_utxo.clone();
+                original_inputs.next();
             }
-            original_inputs.next();
         }
     }
     payjoin_psbt