Steps to reproduce:
1. Download and open the attached project
2. Start baking the lightmap
3. Leave the editor open for a few minutes
OR
2. Build for Standalone

result: crash after some time

Reproduced on 5.6.0f3, 2017.1.0b1
Feature introduced in 5.6

Notes:
- Only occurs in release mode
- It can be consistently reproduced if the build is deployed to a standalone target
- Project scene contains over a thousand individual pieces for which the lightmaps are being built

Verified fixed 5.6.1p2 and 2017.1.0p1