RIP: Modernize packet parsing style.

RIP: Modernize packet parsing style.

Enable ND_LONGJMP_FROM_TCHECK. Report invalid packets as invalid. Remove
two redundant ND_TCHECK_SIZE() instances and an improvised snapshot end
guard. Check bounds for the remaining part of the packet header after
printing version and command, not before. Lose one pointer and one
length variable in rip_print(), also account for the header size when
estimating the number of routes. Update two tests.