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

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 payload do not have any witness. 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 payload and by skipping the witnesses parsing
if that number is not greater than 4 (locktime field length).

Author: hcleonis

bitcoinutils/transactions.py

@@ -567,9 +567,11 @@ def from_raw(rawtxhex: str):
             output, cursor = TxOutput.from_raw(rawtx.hex(), cursor, has_segwit)
             outputs.append(output)
 
-        # Handle witnesses if SegWit is enabled
+        # Handle witnesses if SegWit is enabled and if they are present i.e. if
+        # remaining payload length is greater than last tx field length (locktime)
+        has_witness_field = True if len(rawtx) - cursor > 4 else False
         witnesses = []
-        if has_segwit:
+        if has_segwit and has_witness_field:
             for _ in range(n_inputs):
                 n_items, size = parse_compact_size(rawtx[cursor:])
                 cursor += size