Fixed SDP media transport protocol info parsing

[sauwming]

To fix #3800.

Summary of the report:
In pjmedia_sdp_transport_get_proto():

idx = pj_strtok2(tp, "/", &token, 0);
if (idx != tp->slen)
        pj_strset(&rest, tp->ptr + token.slen + 1, tp->slen - token.slen - 1);

rest.slen will be set to -1 if delimiter / is not found within the string, such as if the transport is plain udp.

This causes the subsequent check within the function to not work properly:

    } else if (pj_stricmp2(&token, "UDP") == 0) {
        /* Starts with "UDP" */

        /* Plain UDP */
        if (rest.slen == 0)
            return PJMEDIA_TP_PROTO_UDP;

Thank you to @miguelpenades for the report and the patch.