UTXO Inventory screen should prevent negative numbers

[leto]

One can make the "Make inventory" button clickable by entering negative numbers even when you don't have enough in total to make inventory

Experienced this on 0.8.8-rc

[satindergrewal]

Thanks for reporting. Condition added to this commit: https://github.com/KomodoPlatform/BarterDEX/commit/948d987e46393a2282b1c7e7c664d9dd1b31ae5a