fix: possible overflow in BuildUnsignedTx (#1303)

* fix: possible overflow

* chore: update changelog

* chore: add comment

CHANGELOG.md

@@ -61,6 +61,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (x/collection) [\#1268](https://github.com/Finschia/finschia-sdk/pull/1268) export x/collection params into genesis
 * (x/collection) [\#1294](https://github.com/Finschia/finschia-sdk/pull/1294) reject NFT coins on FT APIs
 * (sec) [\#1302](https://github.com/Finschia/finschia-sdk/pull/1302) remove map iteration non-determinism with keys + sorting
+* (client) [\#1303](https://github.com/Finschia/finschia-sdk/pull/1303) fix possible overflow in BuildUnsignedTx 
 
 ### Removed
 

client/tx/factory.go

@@ -3,6 +3,7 @@ package tx
 import (
 	"errors"
 	"fmt"
+	"math/big"
 	"os"
 
 	"github.com/spf13/pflag"
@@ -212,7 +213,9 @@ func (f Factory) BuildUnsignedTx(msgs ...sdk.Msg) (client.TxBuilder, error) {
 			return nil, errors.New("cannot provide both fees and gas prices")
 		}
 
-		glDec := sdk.NewDec(int64(f.gas))
+		// f.gas is a uint64 and we should convert to LegacyDec
+		// without the risk of under/overflow via uint64->int64.
+		glDec := sdk.NewDecFromBigInt(new(big.Int).SetUint64(f.gas))
 
 		// Derive the fees based on the provided gas prices, where
 		// fee = ceil(gasPrice * gasLimit).