tinflate.c: Add cast to fix signedness check

[trygvis]

A simple fix, but allows our code to compile with -Wextra and -Werror.

[github-actions[bot]]

Thanks for your submission. However there was no (further) activity on it for some time. Below are possible reasons and/or means to proceed further:

• Please make sure that your submission contains clear and complete description of the issue and information required to understand and reproduce it (exact version of all components involved, etc.). Please refer to numerous guides on submitting helpful bugreports, e.g. https://www.chiark.greenend.org.uk/~sgtatham/bugs.html

• Please make sure that your feature request/suggestion/change aligns well with the project purpose, goals, and process.

• If you face the issue you report while working on a community-oriented open-source project, feel free to provide additional context and information - that may help to prioritize the issue better.

• As many open-source projects, this project is severely under-staffed and under-resourced. If you don't run a community-oriented open-source project, and would like to get additional support/maintenance, feel free to request a support contract.

Thanks for your understanding!

[github-actions[bot]]

Closing due to inactivity.