[Audio] Unity Crashes when AudioMixer's Snapshots are removed

Steps to Reproduce:

1. Open a new project.
2. Create an Audio Mixer
3. Select Audio Mixer and go to Inspector window, change the view from Normal mode to Debug mode.
4. Expand Snapshots array > change array size to 0

Observe that Unity freezes and crashes in a bit

Actual Result:
Unity Crashes when Snapshots array size is changed to 0.

Expected Result:
Exceptions should be handled.

Reproducible on:
2019.1.0a9, 2018.3.0b10, 2018.2.0f2, 2018.1.0f2, 2017.4.15f1, 5.6.6f2

Environment:
Windows 7, Windows 10 and Mac 10.13.