layers: Fix down-chain null funcptr crashes
layers: Fix down-chain null funcptr crashes

Modified dispatch table helper to replace null device extension
function pointers with an appropriate stub function.

Change-Id: If5e22333f084d3f41c0eefdf3e82c9060b872e8c

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