util/sparse_array: don't stomp head's counter on pop operations

util/sparse_array: don't stomp head's counter on pop operations

By temporarily storing the new_head by a uint32_t, we wipe out the
counter section of the head pointer.

Fixes: e4f01eca ("util: Add a free list structure for use with util_sparse_array")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4612>