frontend: Fix SourceButton rename handler

[Warchamp7]

Description

Fix wrong calldata helper.

Motivation and Context

Fixes crash.

How Has This Been Tested?

Compiled OBS and renamed sources.

Types of changes

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

Checklist:

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

[RytoEX]

Looks fine.