negative score for quiz questions clipped

The stock 1.5.2 does not allow negative scores for individual questions or entire quizzes. In other words, it is impossible to use SAT or GRE style point assignments, in which a wrong answer is worth -0.25 points.

A fix is proposed as follows (for 1.5.2):

change line 864 of mod/quiz/locallib.php from

0.0), 1.0) * $question->maxgrade;

to

1.0), 1.0) * $question>maxgrade;