How to reproduce:
1. Create a new project
2. Copy the attached fbx file onto the Assets folder of your project.
3. Select the FBX asset in Unity, go to the animation tab on the Inspector Window.
4. Click on the "Animated Custom Properties" checkbox
5. Click "Apply" button

Actual result: The crash happens due to access to the uninitialized memory when trying to import an FBX file that has animated user properties and multiple clips

Reproduced with: 2017.2.2p1, 2017.3.1p4, 2018.1.0b11, 2018.1.0a4

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Fixed in: 2017.4.1f1, 2018.1.0f1, 2018.2.0a6