UI: Fix cast of pointer type at invalid current scene setting on load

[norihiro]

Description

A variable curScene has a type OBSSourceAutoRelease.
It's pointer &curScene is passed to obs_enum_scenes.
Hence, the right type is OBSSourceAutoRelease *.

Though the first commit 496e599 fixed the bug, there was a complaint that the code is not easy to read. This PR additionally change them for a little better readability of the code.

• Use static_cast instead of reintepret_cast so that some type-checks will be performed at compile time.
• Instead of having a reference type of OBSSourceAutoRelease, put the parameter source_ptr with the cast to the left hand side of the assignment.

Motivation and Context

The code was introduced by #9944.

A crash was reported on the forum. https://obsproject.com/forum/attachments/crash-2024-07-18-09-59-21-txt.105470/

How Has This Been Tested?

OS: Fedora 39

1. Created a new scene collection having one scene named Scene.
2. Manually edit the scene collection file as below and start OBS Studio.

@@ -61,8 +61,8 @@
     "monitoring_type": 0,
     "private_settings": {}
   },
-  "current_scene": "Scene",
-  "current_program_scene": "Scene",
+  "current_scene": "Scene 1",
+  "current_program_scene": "Scene 1",
   "scene_order": [
     {
       "name": "Scene"

Without this fix, I confirmed the crash was reproduced.
With this fix, I confirmed no crash was observed and Scene was selected at the startup.

Also checked with Studio Mode.

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.

[RytoEX]

cc @Penwy @notr1ch

[RytoEX]

To clarify, the forum post is here: https://obsproject.com/forum/threads/obs-crashes-after-setting-up-clock-html-in-scenes-collection.176904/