whois,finger: fix retval, convert from tvb_length

Fixes dissector bug:
failed assertion "save_desegment_offset == pinfo->desegment_offset &&
save_desegment_len == pinfo->desegment_len"

Change-Id: Ia475bea3726252530189a05ba4dbf69f3c2b7e6c
Reviewed-on: https://code.wireshark.org/review/2886
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Evan Huus <eapache@gmail.com>