fix: make Transaction.from_raw() able to parse unsigned segwit tx

[hcleonis]

Although Transaction.from_raw() is able to parse a signed or unsigned legacy tx, it is only able to correctly parse signed segwit txs because it assumes the presence of witnesses in the tx payload. But unsigned segwit tx payloads do not have any witness field.

This PR proposes to fix that by checking the number of remaining bytes between the end of the last output and the end of the tx payload and by skipping the witnesses parsing if that number is not greater than 4 (i.e. last tx field i.e. locktime length).

[karask]

Hi @hcleonis ! Looks good. Can you add a test (in tests) with the example you used to test it? I will then review and merge.

Thank you

[hcleonis]

Hi @hcleonis ! Looks good. Can you add a test (in tests) with the example you used to test it? I will then review and merge.

Thank you

Great! I'm on something else right now but I'll do that as soon as I can.

[boris-on]

Great job, thank you for fix

[hcleonis]

PR rebased onto 0.7.2. Will add a test soon.

[hcleonis]

Hello @karask :wave: I've added a test that validates the unsigned segwit tx parsing with Transaction.from_raw(). It ran OK on my side. I'll let you review & merge as you see fit.