util/hash_table: Properly handle the NULL key in hash_table_u64

util/hash_table: Properly handle the NULL key in hash_table_u64

The hash_table_u64 should support any uint64_t as input.  It does
special handling for the "deleted" key, storing the data in the table
itself; do the same for the "freed" key.

Fixes: b38dab101ca "util/hash_table: Assert that keys are not reserved pointers"
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>