coap_io.c: Stop calls to coap_dtls_handle_timeout() dereferencing the session
coap_io.c: Stop calls to coap_dtls_handle_timeout() dereferencing the session

Wrap calls to coap_dtls_handle_timeout() with coap_session_reference() and
coap_session_release() so that the session pointer can continue to be used.

Found when a DTLS session timed out using OpenSSL.

Fixes CID 1484379

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap