packet-netflow.c: fix bug: "top-of-stack" --> "bottom-of-stack"

Bug #10458
Change-Id: I733bb54b13da8dd5b96837b57c1893e14f9622ec
Reviewed-on: https://code.wireshark.org/review/4067
Reviewed-by: Bill Meier <wmeier@newsguy.com>