Fix fuzzer finding in parse Interger literal

[kimkulling]

• closes https://github.com/kimkulling/openddl-parser/issues/85

Summary by CodeRabbit

• Bug Fixes

  • Fixed a rare crash when parsing numeric values at the end of input, improving stability and preventing undefined behavior.
  • Improved robustness of number parsing without changing parsing outcomes.

• Refactor

  • Minor initialization cleanup for consistency; no functional impact on users.

• Chores

  • Internal code safety improvements with no changes to public APIs.