transaction.deserialize: only set self._inputs after parsing witness

re multi-threaded sanity...
(see lazy deserialization in Transaction.inputs)

electrum/transaction.py

@@ -675,14 +675,15 @@ class Transaction:
             n_vin = vds.read_compact_size()
         if n_vin < 1:
             raise SerializationError('tx needs to have at least 1 input')
-        self._inputs = [parse_input(vds) for i in range(n_vin)]
+        txins = [parse_input(vds) for i in range(n_vin)]
         n_vout = vds.read_compact_size()
         if n_vout < 1:
             raise SerializationError('tx needs to have at least 1 output')
         self._outputs = [parse_output(vds) for i in range(n_vout)]
         if is_segwit:
-            for txin in self._inputs:
+            for txin in txins:
                 parse_witness(vds, txin)
+        self._inputs = txins  # only expose field after witness is parsed, for sanity
         self._locktime = vds.read_uint32()
         if vds.can_read_more():
             raise SerializationError('extra junk at the end')