Fix potential use-after-free and memory leak

In function wait_for_async(), allocated async fds is freed if
`SSL_get_all_async_fds` fails, but later `fds` is used. Interestingly,
it is not freed when everything succeeds.

Rewrite the FD set loop to make it more readable and to not modify the allocated
pointer so it can be freed.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/3992)