Fix PSK handling.

Fix PSK handling.

The PSK identity hint should be stored in the SSL_SESSION structure
and not in the parent context (which will overwrite values used
by other SSL structures with the same SSL_CTX).

Use BUF_strndup when copying identity as it may not be null terminated.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(cherry picked from commit 3c66a669dfc7b3792f7af0758ea26fe8502ce70c)