fix: possible overflow in BuildUnsignedTx

[jaeseung-bae]

Description

• (audit issue) fix possible overflow in BuildUnsignedTx
  • referenced implementation(https://github.com/cosmos/cosmos-sdk/pull/18622)

Motivation and context

How has this been tested?

Screenshots (if appropriate):

Checklist:

• [x] I followed the contributing guidelines and code of conduct.
• [x] I have added a relevant changelog to CHANGELOG.md
• [ ] I have added tests to cover my changes.
• [ ] I have updated the documentation accordingly.
• [ ] I have updated API documentation client/docs/swagger-ui/swagger.yaml

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.43%. Comparing base (c462f36) to head (40ef613).

Additional details and impacted files

[](https://app.codecov.io/gh/Finschia/finschia-sdk/pull/1303?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Finschia) ```diff @@ Coverage Diff @@ ## main #1303 +/- ## ======================================= Coverage 70.43% 70.43% ======================================= Files 643 643 Lines 54720 54720 ======================================= Hits 38542 38542 Misses 14008 14008 Partials 2170 2170 ``` | [Files](https://app.codecov.io/gh/Finschia/finschia-sdk/pull/1303?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Finschia) | Coverage Δ | | |---|---|---| | [client/tx/factory.go](https://app.codecov.io/gh/Finschia/finschia-sdk/pull/1303?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Finschia#diff-Y2xpZW50L3R4L2ZhY3RvcnkuZ28=) | `59.88% <100.00%> (ø)` | |