refactor: ensure that the transaction amount is an unsigned integer (#67)

Author: Pedro-Souza

crypto/transactions/builder/transfer.py

@@ -17,7 +17,10 @@ def __init__(self, recipientId, amount, vendorField=None, fee=None):
         """
         super().__init__()
         self.transaction.recipientId = recipientId
-        self.transaction.amount = amount
+        if type(amount) == int and amount > 0:
+          self.transaction.amount = amount
+        else:
+          raise ValueError('Amount is not valid')
         self.transaction.vendorField = vendorField.encode() if vendorField else None
         if fee:
             self.transaction.fee = fee