Set default advanced tab gas limit

The advanced tab of the transaction confirmation screen would enter
into an infinite loop and crash if the given gas price was falsy and
some interaction was made with the gas limit field.

To prevent this infinite loop, a default value of 0 has been set. The
user will still need to update the gas limit in order to confirm the
transaction, as zero is too low a gas limit (the lowest is 21000).
21000 cannot be the default gas limit at this layer, because the limit
used is from a layer above this, which wouldn't have that same 21000
set.

ui/app/components/app/gas-customization/advanced-gas-inputs/advanced-gas-inputs.component.js

@@ -11,8 +11,8 @@ export default class AdvancedGasInputs extends Component {
   static propTypes = {
     updateCustomGasPrice: PropTypes.func,
     updateCustomGasLimit: PropTypes.func,
-    customGasPrice: PropTypes.number,
-    customGasLimit: PropTypes.number,
+    customGasPrice: PropTypes.number.isRequired,
+    customGasLimit: PropTypes.number.isRequired,
     insufficientBalance: PropTypes.bool,
     customPriceIsSafe: PropTypes.bool,
     isSpeedUp: PropTypes.bool,

ui/app/components/app/gas-customization/advanced-gas-inputs/advanced-gas-inputs.container.js

@@ -12,7 +12,7 @@ function convertGasPriceForInputs (gasPriceInHexWEI) {
 }
 
 function convertGasLimitForInputs (gasLimitInHexWEI) {
-  return parseInt(gasLimitInHexWEI, 16)
+  return parseInt(gasLimitInHexWEI, 16) || 0
 }
 
 const mapDispatchToProps = dispatch => {