libobs: Fix holding possibly released pointer in obs_output_t

When `obs_output_t` is an encoded output, `obs_output_set_media` will
ignore the video and audio so that `obs_output_t` will keep holding the
`video_t` and `audio_t` pointer when the output was created.
By this commit, `video` and `audio` member variables in `obs_output_t`
will never set if it is an encoded output.
In the auto-configuration wizard, `video_t` is released and created to
have a different video size while `obs_output_t` is not released. This
resulted in accessing the released `video_t` pointer.
This commit is contained in:
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f