Fix an infinite loop when the line has no length

Bug:10333
Change-Id: Ia2dae94095dddf57e4c75792b5477b0eb37c29c9
Reviewed-on: https://code.wireshark.org/review/3345
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-on: https://code.wireshark.org/review/4423
Reviewed-by: Balint Reczey <balint@balintreczey.hu>
Tested-by: Balint Reczey <balint@balintreczey.hu>