CVE-2017-13048/RSVP: fix decoding of Fast Reroute objects

In rsvp_obj_print() the case block for Class-Num 205 (FAST_REROUTE) from
RFC 4090 Section 4.1 could over-read accessing the buffer contents before
making the bounds check. Rearrange those steps the correct way around.

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).