elm fileselector: fix possible premature deletion of Listing_Request struct

elm fileselector: fix possible premature deletion of Listing_Request struct

In a case where eina_promise_then is executed immediately (like with some
quick and light Efl.Model), the Listing_Request struct will be prematurely
freed in the first iteration of the child processing loop, because the
item_total counter had not accumulated the right number of items yet.

With this commit, we traverse the children accessor first, so we can know
the number of items.

Also, no longer use the Listing_Request pointer after the loop, once it
may have been deallocate already.
And put a note about this too.