loader: Fix size handling when scanning registries
loader: Fix size handling when scanning registries

loaderGetRegistryFiles() should not start from 4096 size, if
loaderGetDeviceRegistryFiles function has already appended something
into the buffer.

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