Fix nil pointer exception in txList Filter

This occurred because gasPriceMinimums contains the gasPriceMinimum for
every fee currency, not just the fee currencies of the transfer. This
changes the iteration to go through each gasPriceFloor and then get the
gasPriceMinimum from the fee currency associated with the floor. This works
because gasPriceMinimums contains every whitelisted fee currency and the tx
has already been validated to pay in a fee currency.

core/tx_list.go

@@ -371,8 +371,8 @@ func (l *txList) Filter(nativeCostLimit, nativeGasPriceMinimum *big.Int, feeLimi
 		}
 	}
 	// Ensure that each gas price floor >= the gas price minimum.
-	for feeCurrency, gasPriceMinimum := range gasPriceMinimums {
-		if l.gaspricefloors[feeCurrency].Cmp(gasPriceMinimum) < 0 {
+	for feeCurrency, gasPriceFloor := range l.gaspricefloors {
+		if gasPriceMinimum := gasPriceMinimums[feeCurrency]; gasPriceFloor.Cmp(gasPriceMinimum) < 0 {
 			canBail = false
 			l.gaspricefloors[feeCurrency] = new(big.Int).Set(gasPriceMinimum)
 		}