Negative gas price breaks the extension

[sterlu]

Describe the bug
When signing a transaction, entering a negative gas price will break the extension. The tx confirmation window will freeze and opening the UI will render an empty screen. In the console, a BigNumber constructor error message will be displayed due to it being created with a negative hex value.

I've eventually worked around this by disabling the extension, switching to Kovan, entering the password to log in, opening settings, and reseting the account.

To Reproduce

1. Create a transaction to send (for example, send 0 ETH to yourself)
2. On the tx confirmation screen, with the gas price in focus, tap the Down arrow on the keyboard to decrease the price, going below 0
3. The UI should presend an infinitely loading overlay; closing the pop-up and opening the UI again should render a blank screen

Expected behavior
Gas price input should prevent the user from entering a negative value.

Browser details (please complete the following information):

• OS: Mac OS
• Browser: Chrome 81.0.4044.129 (Official Build) (64-bit)
• MetaMask Version: 7.7.8