ISM Component in an instanced BP does not take into account changes to NumCustomDataFloats from the BP class

PerInstanceSMCustomData is bulk serialized and doesn't get changed on the BP instance when changing NumCustomDataFloats, possibly causing out of bounds array access leading to a crash.

Possible solution is to fix up this mismatch in UInstancedStaticMeshComponent::PostLoad()

1. Create a BP Class containing an Instance Static Mesh Component with one instance. 
2. Set NumCustomDataFloats to 1 in on the Instance Static Mesh Component in the BP. Save.
3. Place the BP in a new level and save it
4. Unload that level (jump into some other level)
5. Go back to the BP class and change NumCustomDataFloats to 2. Save.
6. Load the newly created level