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>