util/set: Assert that keys are not reserved pointers

util/set: Assert that keys are not reserved pointers

If we insert a NULL key, it will appear to succeed but will mess up
entry counting.  Similar errors can occur if someone accidentally
inserts the deleted key.  The later is highly unlikely but technically
possible so we should guard against it too.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>