fix memory leaks

[norihiro]

Description

Correctly release allocated memories.
Also skip processing the shader if gs_effect_create returns NULL.

Motivation and Context

This is a same PR as exeldro#1.

How Has This Been Tested?

Tested on my custom build on Linux (Fedora 34).
To compile on Linux, I locally modified CMakeLists.txt.

Preparation:

1. Start OBS
2. Add the filter to a scene.
3. Exit OBS

When testing:

1. Start OBS with a scene collection that has the filter attached to a scene
2. Exit OBS
3. Check the number of memory leaks.

The log shows no memory leaks as below.

info: Number of memory leaks: 0

Types of changes

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