Increase space allocated for IO Graph slower after 1 million items

Change-Id: I691070b572e519f412a80db5d7415331f07e8e50
Reviewed-on: https://code.wireshark.org/review/2689
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Reviewed-by: Balint Reczey <balint@balintreczey.hu>