visual: fix memory-related issues in calculating file percentage

[PeiweiHu]

Your checklist for this pull request

• [x] I've read the guidelines for contributing to this repository
• [x] I made sure to follow the project's coding style
• [ ] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• [ ] I've added tests that prove my fix is effective or that my feature works (if possible)
• [ ] I've updated the rizin book with the relevant information (if needed)

Detailed description

fix:

https://github.com/rizinorg/rizin/pull/4536#issuecomment-2151893276

Test plan

CI is green.

Closing issues

...