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.

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• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
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Please don't

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[github-actions]

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Name	fkYAML_coverage.pr355.zip
ID	1560427092
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[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%

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

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