UDAF may return bad value when it should return NULL

During final evaluation of UDAF, if the result should be NULL and if, in that particular thread, a non-NULL value has already been created for another row (because of GROUP BY), that other calculated number is returned, rather than NULL.

This is because the variable used to store the result is not reset inside the loop.

During execution, this is unpredictable because one never knows which rows will be evaluated in which threads.