UI: Fix crash in Source Toolbar

[Chiitoo]

I guess I might as well push this out here, at least as a draft, for a bit of review and suggestions for improvement.

Description

This adds a check for 'null' in 'mod' and simply 'return' if it's there, to prevent a segmentation fault.

Motivation and Context

This prevents a segmentation fault when, for example, an ALSA source is selected, and adds ALSA devices to the Source Toolbar (this could most likely be done better).

This also fixes the following issue: #3485

How Has This Been Tested?

Tested building and running OBS Studio with all sound backends disabled, as well as with both, or only one of ALSA and PulseAudio enabled on Gentoo Linux.

I don't actually ever use PulseAudio, so more testing with that would be nice. I can only say that I saw more sources available with it installed.

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.

[dodgepong]

These should be two separate pull requests, IMO.

[Chiitoo]

I did consider that as well, but, for one, either of them fixes the mentioned issue for me, so I probably couldn't decide which one should be marked as fixing it. :]

If the first commit seems good, the second can be safely dropped, and I'll create another PR for that (if it looks sane enough for that).

Thanks!

[kkartaltepe]

This makes alsa the default audio monitoring backend which is not appropriate.

[Chiitoo]

Right, it seemed to me like that part should do something different, but PulseAudio also seems to work even if ALSA is enabled as well.

That said though, like I mentioned in the initial comment, I don't actually use PulseAudio, so I'm not sure if it actually works (I have one user report about it working though).

It should not mess with any defaults per se, unless I'm missing something (which is entirely possible).

[Chiitoo]

I split the ALSA support part off since indeed, it might not be ready for a while, and this change alone already prevents the crash (the toolbar just doesn't say anything).

I do wonder a bit if it would make sense to use 'Basic.Settings.Audio.UnknownAudioDevice' there, but not sure it's really worth it.

[kkartaltepe]

Looks good