frontend: Use static_cast when casting from void pointers

[cg2121]

Description

Using static_cast is preferred here, as it is safer to use than reinterpret_cast.

Motivation and Context

Better code.

How Has This Been Tested?

Compiled and ran OBS to made sure nothing broke.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.