[bisq-network/bisq] Prevent transient negative missingCoin amount sent to QRCode display (#4258)

[James Cox]

### Testing

To reproduce the error: create an offer and fund it from your external wallet.  If you send more than the required amount, an error message will pop up.

![image](https://user-images.githubusercontent.com/47253594/82002539-a422fe80-9623-11ea-8c3d-8ee0cf582125.png)

### Verification

Make the same test (overfund your trade from an external wallet).  With the fix, no error message will be displayed, and the QRCode will indicate the amount remaining = 0.

![image](https://user-images.githubusercontent.com/47253594/82002688-0f6cd080-9624-11ea-92d9-91cada6b9aef.png)




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