psbt: reject trailing final witness data

[starius]

Change Description

Follow-up for #2558

Found one more case where an input with trailing bytes was accepted: final witness data in psbt.Extract.

Steps to Test

go test -run TestExtractRejectsTrailingFinalScriptWitnessData -count=1 -v

then revert the fix commit and make sure it is failing:

=== RUN   TestExtractRejectsTrailingFinalScriptWitnessData
      psbt_test.go:827:
                Error Trace:    /tmp/btcd-bip322-revert-msg/psbt/psbt_test.go:827
                Error:          Target error should be in err chain:
                                expected: "Invalid PSBT serialization format"
                                in chain:
                Test:           TestExtractRejectsTrailingFinalScriptWitnessData
  --- FAIL: TestExtractRejectsTrailingFinalScriptWitnessData (0.00s)

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[guggero]

Nice, thanks for the fix! LGTM 🎉