loader: Set pointer to NULL
loader: Set pointer to NULL

Cleanup enabled extension array pointer passed down to driver.
If the loader overrode the create info struct, the pointer
could be garbage if the enabled extension count was 0.  Now,
make sure it is NULL.

Change-Id: I558d768e786892e7afc44f58b4173a2b8db3deee

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