[Bug bootstrap/92762] hash_table::empty_slow invokes assignment on invalid objects

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92762

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
This bug is distinct from pr92761 because it's in hash_table::empty_slow() and
pr92761 is in hash_table::expand().  I expect to post a patch for both so I'm
fine with handling them together.