Reproduction steps:
1. Open the attached project (871719-TestProject.zip)
2. Select the texture in Project dialog
3. Select Texture type = Normal map, check Create from Grayscale, and hit apply
4. Uncheck Create from Grayscale and hit apply
5. Check Create from Grayscale and hit apply
6. Then repeat the steps 4 and 5 till the Unity will crash
4. Observe, the crash

Expected result: Unity should not crash
Actual result: Unity crashes

Reproduced with: 5.3.7p3, 5.4.4p1, 5.5.0f3, 5.5.0p4, 5.6.0b4