Incorrectly reporting `zend` error if wallet balance is exactly 0.007 regardless of number of wallet UTXOs

[Jordan Ellis Coppard]

If a node's shielded address contains UTXOs for a balance above the minimum 0.001, once that balance reaches 0.007 it will incorrectly, and indefinitely, report zend exceptions every 6 minutes when the tracker requests the z_addr balance.

This can be fixed by using a new wallet.dat, or by forcing a challenge so that the balance then becomes 0.0068.

This has happened numerous times for nodes where I have sent 8 payments of 0.001 to a nodes shielded challenge address.