Reimporting skeletal mesh clears material references from material slots

This occurs in //UE4/Release-4.15, //UE4/Dev-Editor, and //UE4/Main

Does not occur in 4.14.3

1. Import a skeletal mesh via FBX
2. Right click the skeletal mesh asset and click Reimport

Result: Observe that upon reimport, all of the material assignments in the material slots are cleared out

Expected: Material assignments should be maintained on reimport