loader: Fix pre-instance proc addresses
loader: Fix pre-instance proc addresses

Fix a bug where vkGetInstanceProcAddr would return a null pointer for
any command that does not require an instance if an instance was
given

Change-Id: Iec5766f9c37104c36852f758d391a9b0b54ec0ec

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers