coap_io.c: Prevent coap_write() blowing up if opaque tls is not set
coap_io.c: Prevent coap_write() blowing up if opaque tls is not set

When determing whether to do a re-transmit timeout based on
coap_dtls_get_timeout, if s->tls is not set (unable to set up TLS session,
TLS error etc.), then coap_write() assert's when calling
coap_dtls_get_timeout().  This fix just sets the timeout to 0 if s->tls is not
set.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap