When GI is set to "Auto", calculating the ambient skybox probe while the skybox material intensity slider is being moved can cause Unity to hang or crash.

1. Open Scene1 from attached project
2. Select New Material 1 from the project folder
3. In the inspector, start moving the Exposure slider down and back up again
4. Unity crashes or hangs.

Expected outcome: Unity doesn’t crash :)

Notes:
- crash also occurs when dragging the rotation slider.
- seems to be happening more frequently with larger skybox files.
- managed to reproduce with procedural skybox material, albeit not as often.
- tested on MacBook Pro (Retina, 15-inch, Mid 2015), OS X El Capitan (10.11.2)
- can't repro in Win10.
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- regression, works in:
Version 5.4.0b7 (83df6fa5e23f)
Tue, 16 Feb 2016 17:18:13 GMT
Branch: 5.4/staging
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- crash occurs in:
Version 5.5.0a1 (5a6d256ca545)
Tue, 16 Feb 2016 14:06:25 GMT
Branch: graphics/staging

Version 5.5.0a1 (43a8bf21b0d6)
Mon, 15 Feb 2016 09:22:25 GMT
Branch: trunk