Account required should be checked when skipping signature verification in join_splits

[sirasistant]

The join_split circuit now skips signature verification for merge join_splits since It checks that the spender and total amounts are going to remain the same and that only note aggregation is happening.

However, using the account_required flag an exploiter that has control over an account key but no spending keys could still drain the notes that have account_required to true:

• Decode the notes using the account key
• Do regular merge join_splits with them, with a fake signature (since it's going to be skipped) to the same owner BUT creating the output notes as account_required = false
• Now the attacker can spend the notes with the account key since the output notes are account_required false

[ludamad]

Great catch

[LeilaWang]

Closed by #70