CVE-2017-13051/RSVP: fix bounds checks for UNI

Fixup the part of rsvp_obj_print() that decodes the GENERALIZED_UNI
object from RFC 3476 Section 3.1 to check the sub-objects inside that
object more thoroughly.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s).
rsvp_clear_checksum(void *header)
{
    struct rsvp_common_header *rsvp_com_header = (struct rsvp_common_header *) header;

    rsvp_com_header->checksum[0] = 0;
    rsvp_com_header->checksum[1] = 0;
}

rsvp_clear_checksum(void *header)
{
    struct rsvp_common_header *rsvp_com_header = (struct rsvp_common_header *) header;

    rsvp_com_header->checksum[0] = 0;
    rsvp_com_header->checksum[1] = 0;
}
