libobs: Fix crash when mix is NULL

[exeldro]

Description

When obs_reset_video is called, it calls obs_free_video that sets the mix to NULL here: https://github.com/obsproject/obs-studio/blob/master/libobs/obs.c#L879

Motivation and Context

Don't like crashes when switching resolution

How Has This Been Tested?

On windows by changing the resolution multiple times

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.

[tt2468]

Just curious, in what situation did you encounter this?

[exeldro]

I encountered this investigating a bug report with Aitum Vertical causing a crash after switching resolution of the main canvas

[notr1ch]

In obs_free_video , wouldn't including the da_free(obs->video.mixes); in the mutex be a better solution?

[exeldro]

That should work too, I have no preference

[notr1ch]

I think I'd prefer that as it protects against other possible race conditions.