Steps to reproduce:
1. Open attached project
2. Open "Peli" Scene
3. Select "walrus" in Hierarchy window
4. Open Animation window
5. Extrude "walrus : Position"
6. Press on little rectangle next to the Position.x and select "Remove properties"
7. CTRL + Z
-Invalid OffsetPtr access! Pointer is NULL

Expected result: no error should be thrown when making undo operation
Actual result: Invalid OffsetPtr access! Pointer is NULL is thrown

Reproduced with: 5.6.0f3, 2017.1.0a6
Regression found in: 5.5.2p2
Crashed when trying to click on "walrus" game object in 5.6.0a6, 5.6.0b4, 5.6.0f1