obs-browser: Ensure saved source is alive

[walker-WSH]

Description

In browser-client, created source is saved into two lists:

std::vector<obs_source_t*> audio_sources;
std::unordered_map<int, AudioStream> audio_streams;

But there is no reference for audio_sources. I meet a crash as below:

Motivation and Context

How Has This Been Tested?

Types of changes

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.