fix memory leak in Fst2List

[JM-ERI]

Description

Solve the issue #87.
resolve a memory leak in the current solution.

Motivation and Context

Without this change, the output file of explore graph paths is not readable for Korean users.
It fixes a memory leak that occurs in the code

How Has This Been Tested?

This change has been tested on some graphs in korean.
korean_tests_graphs.zip
tested locally with unitex-core-test.

Screenshots (if appropriate):

Type of files

• bin: Binary files
• ci: Continuous integration files
• doc: Documentation files
• Plain-text source code files

Level of change

• break: Breaking change
• exp: Experimental change
• tmp: Temporal change
• major: Major change
• minor: Minor change
• sec: Vulnerability-related change
• None of the above (normal change)

Type of change

• deprecat: Deprecation of a once-stable feature
• enhance: Enhancement in existing functionality
• fix: Bug fix
• feature: New feature
• hotfix: Hotfix for bugs
• refactor: Improve coding style, comments
• remove: Remove a feature

Checklist:

• My code compiles
• My code follows the code style of this project
• My code includes javadoc/doxygen where appropriate
• My code is well factored, so that there is not repetitive code in the wild
• My code does not require a change in the documentation, if so I already opened an issue to list the changes
• I have read the CONTRIBUTING document
• I have read the Commit Message Guidelines
• I understand that all commits on my pull request will be squashed to a single good one

[martinec]

LGMT