vk-layer-generate.py: assert valid instance/device in vkGet*ProcAddr
vk-layer-generate.py: assert valid instance/device in vkGet*ProcAddr

"device" should be always valid in vkGetDeviceProcAddr.  "instance" should
be valid in vkGetInstanceProcAddr when the command is not intercepted by
intercept_core_instance_command.

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