message: [qa] calc hash on prevtx in create_transaction
  fixes issue where calculating the hash of the created transaction fails
  with obscure error message
diff: @@ -75,6 +75,7 @@ def create_coinbase(height, pubkey = None):
  # or a list to create multiple outputs
  PADDED_ANY_SPEND = b'\x61'*50 # add a bunch of OP_NOPs to make sure this tx is long enough
  def create_transaction(prevtx, n, sig, value, out=PADDED_ANY_SPEND):
  + prevtx.calc_sha256()
  if not type(value) is list:
  value = [value]
  tx = CTransaction()
  