Move the call to destroy the PSK to before freeing the SSL session
data and calling rng_free(), which deinitializes the PSA subsystem.
This particular ordering was chosen to make the ssl_client2 more
similar to ssl_server2. This fixes the client failing on the
psa_destroy_key() call in `ssl-opt.sh -f 'opaque psk on client'`.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>