Don't treat going past the end of the buffer in check_add_path() as an error.

Don't treat going past the end of the buffer in check_add_path() as an error.

It's just a heuristic test to try to guess whether the NLRI information
has path IDs or not.  If we run past the end of the packet data, just
stop processing before we try to fetch data past the end, don't return
an error.

This keeps us from showing truncation in non-truncated packets.

Update tests to reflect that change.