linux-alsa: Fix use-after-free crash when USB mic not plugged in

[R0te]

Description

This change fixes a use-after-free crash in the ALSA input plugin that occurs when creating a new scene collection without the previously configured USB microphone plugged in. A shutdown flag is introduced to safely coordinate the termination of background threads and prevent access to freed resources.

Motivation and Context

Previously, _alsa_reopen() could run concurrently after alsa_destroy() freed the device string or other resources, causing a heap-use-after-free crash. This fix ensures that background threads properly exit when the source is destroyed, eliminating the race condition.
#9322

How Has This Been Tested?

• Started OBS without the USB mic plugged in.
• Created a new Scene Collection.
• Switched to the new Scene Collection and waited a few seconds.
• Confirmed that OBS no longer crashes and that no AddressSanitizer errors are reported.

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.

[Lain-B]

Volatile variables should probably be used with the atomic functions. And if the other volatiles aren't being used with atomic operations, they probably should be.