tapfreighter: unlock asset coins before err on tx broadcast fail

Ensure locked asset coins are released before returning an error when
attempting to publish the anchor transaction. Without this change, if
the chain bridge logic determines the anchor transaction should not be
broadcast, the coins remain locked, leaving the send attempt stuck in
limbo.

tapfreighter/chain_porter.go

@@ -1391,6 +1391,10 @@ func (p *ChainPorter) stateStep(currentPkg sendPackage) (*sendPackage, error) {
 				"transaction %v: %w", txHash, err)
 
 		case err != nil:
+			// Unlock the inputs we locked for this transfer before
+			// returning the error.
+			p.unlockInputs(ctx, &currentPkg)
+
 			return nil, fmt.Errorf("unable to broadcast "+
 				"transaction %v: %w", txHash, err)
 		}