Fix parse error on smal negative integer keys

[fktn-k]

The current parser mistakenly emits parse errors on small negative integer keys like the following valid YAML snippet:

# This causes a false error which tells there is an invalid ending of a flow mapping.
{-4: bar}

This was caused by the wrong lookahead implemented for tokens which start with a hyphen(-).
So, this PR has fixed it and the above snipet is now parsed successfully.

---

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions[bot]]

:octocat: Upload Coverage Event Notification

Coverage data has been uploaded for the commit 46e831e3404672788f3044e34f3c20c9a0125b14. You can download the artifact which contains the same file uploaded to the Coveralls and its HTML version.

Name	fkYAML_coverage.pr355.zip
ID	1560427092
URL	https://github.com/fktn-k/fkYAML/actions/runs/9340029437/artifacts/1560427092

[coveralls]

Pull Request Test Coverage Report for Build 9340029437

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 9339279478:	0.0%
Covered Lines:	3901
Relevant Lines:	3901

---

💛 - Coveralls