hashtable: properly handle insert() de-set-ifying

GHashTable remains a set for as long as all of the keys are exactly
equal (in pointer value) to all of the values.  We check this by
comparing keys to values when we do inserts.

Unfortunately, when doing g_hash_table_insert() when a key is already in
the table, the old key pointer value is kept, but the new value pointer
is used.  Now we have a situation where a key pointer is unequal to a
value pointer, but we were not treating this case properly.

Fix that.

https://bugzilla.gnome.org/show_bug.cgi?id=692815