Memory stomp in UTexture2DArray::SetPlatformData

A crash from bad memory access can happen when clearing the textures in a Texture2DArray asset.

This appears to be because the order of operations when deleting the platform data is:
```
if (PrivatePlatformData)
{
delete PrivatePlatformData;
if (PrivatePlatformData)

PrivatePlatformData = nullptr;
}
```

This comes from the functions `UTexture2DArray::InvalidateTextureSource()` and `UTexture2DArray::SetPlatformData(...)` from Texture2DArray.cpp.

1. Download the Licensee's test project (see addl info URL)
2. Launch the text project with the command-line argument "-stompmalloc"
3. Open /Game/NewTexture2DArray
4. Add the texture /Game/Black to the Source Textures
5. Clear the Source Textures via the Remove All Elements button
6. Observe the memory access exception.