tn5250: fix an endless loop, exit when offset is not incremented

Bug: 10596
Change-Id: I64509aa49fce4893dee4e7a74a560e09a98830f6
Reviewed-on: https://code.wireshark.org/review/4878
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>
(cherry picked from commit bc2726578156f3608960fc65ce1f691639e6addc)
Reviewed-on: https://code.wireshark.org/review/4879
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-on: https://code.wireshark.org/review/5338
Reviewed-by: Michael Mann <mmann78@netscape.net>