This fixes a bunch of new CTS tests which look for exactly this.  Even in
the cases where we just call vk_free to free a CPU data structure, we still
handle NULL explicitly.  This way we're less likely to forget to handle
NULL later should we actually do something less trivial.

Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>