Make sure we don't set the snapend before the beginning of the packet.

Make sure we don't set the snapend before the beginning of the packet.

If a caller attempts to set it after the current snapend, just silently
ignore the attempt.

If they try to set it before the beginning of the packet, report it as a
bug and quit dissection.  Add a new setjmp() return value meaning "bug"
rather than "truncated packet", add an "nd_bug_longjmp()" routine for
reporting bugs where we should quit dissecting, and use that in this
case.