Fix memory leak in sentence_update_line_no when relabeling menu items

[Abhishekmishra2808]

Summary

There is a memory leak in the menu relabeling loop inside sentence_update_line_no.
Allocated strings are not always freed on early continue paths, and one allocated label is not freed after use.

Affected code

• Allocation of file_name: 389 src/sentence.c
• Early continue after parse failure: 393 src/sentence.c
• Early continue after line check: 396 src/sentence.c
• Allocation of new_label: 405 src/sentence.c
• Label set call: 411 src/sentence.c

Problem details

In the loop:

• file_name is allocated, but if parsing fails or line number checks skip the entry, execution continues without freeing file_name.
• new_label is allocated and passed to gtk_menu_item_set_label, but is not freed afterward.

This causes repeated small leaks during UI operations that update sentence numbering.

Proposed fix

• Free file_name before each continue that exits the current iteration.
• Free new_label after gtk_menu_item_set_label.
• Add a guard for null label before strlen to avoid potential crash in edge cases.

Potential benefits

• Reduces cumulative memory usage during long editing sessions.
• Improves runtime stability and predictability.
• Low-risk, localized fix with minimal code change.
• Improves maintainability by reducing noise in memory diagnostics.

[Abhishekmishra2808]

@kovzol I found this while exploring the repository and wanted to raise it because it looks like a practical, low-risk fix.
I am planning to work on it since it should reduce cumulative memory usage over long editing sessions and improve runtime stability.