RSVP: fix display of unknown Juniper Attribut

Get "Trying to fetch an unsigned integer with length" when length > 4

Change-Id: If5e53b826d98f2c30253ea852754f856e1d29088
Reviewed-on: https://code.wireshark.org/review/6038
Reviewed-by: Anders Broman <a.broman58@gmail.com>