json-parser: Fix cloning when no marker is set

When cloning a json-parser, only clone the marker if it is set in the
original, otherwise we may end up crashing trying to strlen(NULL).

Reported-by: Fabien Wernli <bugzilla.balabit@faxm0dem.org>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>