Author: Tim Hunt
MDL-38279 questions: must have default mark positive.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=a78c180efaca54b165a4223fdf5a1cd06897adcb
http://github.com/moodle/moodle/commit/a78c180efaca54b165a4223fdf5a1cd06897adcb
+ 2.4 en [defaultmarkmustbepositive,core_question]