How to reproduce:
1. Open attached Unity project "MeshColliderBug"
2. Open the MeshColliderBug scene
3. Select MediumPolySphere in the Inspector window
4. Note that it has a polygon mesh in the scene view
5. Right-Click on the Mesh Collider Bug (Script) in the Inspector window
6. Select "0) Set Original Mesh"
7. Select "1) Set broken Mesh On Collider"
8. Note that the mesh disappears
9. Select "2) Set Mesh On Collider"
10. Note that, even though a new mesh has been set, the collider is still broken

Expected result: Setting a new mesh 'fixes' the mesh collider.
Actual result: Collider is still broken after setting new mesh.

Reproduced with: 2018.1.0b6, 2017.3.1f1, 2017.2.1p3, 2017.1.3p1, 5.6.5p1.

Fixed in: 2019.2.0a5, 2019.1.0b5, 2018.3.9f1.