Steps to reproduce:
1. Open attached project's scene "Terrain test".
2. Set one terrain's Y to -50.
3. Wait for lightmapping to finish.
4. Unity crashes.

Or:
1. Create new project.
2. Create two terrains, set one's Y to -50.
3. Wait for lightmapping to finish.
4. Unity crashes.

Reproduced on Windows 7 x64 with 5.1.1f1 (2046fc06d4d8), 5.1.1p2 (82544090e14f), 5.2.0a4 (7747a2c50081)

Seems to be fixed in 5.1.2p1 (4b35e09f2de6).

No repro on 4.6.6p4 (d61021eb48b3)