ossl_rio_poll_builder_add_fd(): Fixup pfds after reallocation

ossl_rio_poll_builder_add_fd(): Fixup pfds after reallocation

Local variable `pfds` used in `ossl_rio_poll_builder_add_fd()` must be consistent
with `rpb->pfd_heap`. The function maintains array of SSL objects for SSL_poll(3ossl).
It works with no issues until we need to reallocate `rbp->pfd_heap` in `rpb_ensure_alloc()`.
After `rpb_ensure_alloc()` returns we must update local variable `pfds` with `rpb->pfd_heap`
not doing so makes function to write to dead buffer.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27804)