layers: Fix Windows crash in Dota2
layers: Fix Windows crash in Dota2

In core_validation attachment_references_compatible(), having array
elements set to VK_ATTACHMENT_UNUSED was not handled resulting in
invalid memory references.

Change-Id: If78aea1467d191db905272bece16cc9f56643136

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools