hash table: enforce testing is_empty before is_deleted

hash table: enforce testing is_empty before is_deleted

Existing hash_table traits that use the same representation for empty
and deleted slots reject marking slots as deleted, and to not pass
is_deleted for slots that pass is_empty.

Nevertheless, nearly everywhere, we only test for is_deleted after
checking that !is_empty first.  The one exception was the copy
constructor, that would fail if traits recognized is_empty slots as
is_deleted, but then refused to mark_deleted.

This asymmetry is neither necessary nor desirable, and there is a
theoretical risk that traits might not only fail to refuse to
mark_deleted, but also return is_deleted for is_empty slots.

This patch introduces checks that detect these potentially problematic
situations, and reorders the tests in the copy constructor so as to
use the conventional testing order and thus avoid them.

for  gcc/ChangeLog

	* hash-table.h (is_deleted): Precheck !is_empty.
	(mark_deleted): Postcheck !is_empty.
	(copy constructor): Test is_empty before is_deleted.