Only fake an item by returning 'tree' if they use the same TVB

Otherwise the offsets will be wrong when calling, e.g. proto_item_set_end.

Bug:10329
Change-Id: I5ae0c660af90678e446817b65f83c075e7c0b84e
Reviewed-on: https://code.wireshark.org/review/3341
Reviewed-by: Evan Huus <eapache@gmail.com>