CVE-2017-12985/IPv6: Check for print routines returning -1 when running past the end.

rt6_print(), ah_print(), and esp_print() return -1 if they run up
against the end of the packet while dissecting; if that happens, stop
dissecting, don't try to fetch the next header value, because 1) *it*
might be past the end of the packet and 2) we won't be using it in any
case, as we'll be exiting the loop.

Also, change mobility_print() to return -1 if it runs up against the
end of the packet, and stop dissecting if it does so.

This fixes a buffer over-read discovered by Brian 'geeknik' Carpenter.

Add tests using the capture files supplied by the reporter(s).