vkGetInstanceProcAddr must return NULL when the instance parameter is
NULL, except for a few global functions.

vkGetDeviceProcAddr must return NULL for any non-device-level functions.

Fixes dEQP-VK.api.version_check.entry_points

Bug b/116336664

Change-Id: I446edb0b8c4b94938e0ab77913c8ad391d490f4f
Reviewed-on: https://swiftshader-review.googlesource.com/c/22688Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>