Crash if txn amount + fee = more than addresses contain

[gdvine]

OS X v0.95

Steps to reproduce:

1. Select "Send"
2. Use Coin Control to select an address
3. Select "MAX" for txn amount
4. Change fee to higher amount
5. Click Continue button

Expected result:
Amount automatically reduces to accomodate new fee OR
Warning popup informs user the total transaction amount is too high

Actual result:
Spinning wheel crash

[Douglas Roark]

Psst. Please close this out and go to the new repo (and the new web site, and install 0.96.4 RC2).