Fix mem-set: for aggregates, pointer is incremented by offset

Recently added test struct-values.translation.mem-aref.1 that tested
conversion of arrays of structures was failing because mem-set did not
increment the pointer by the offset amount, so it was always writing
into the first position of the array and the rest was garbage.  This
is now fixed.  

(with-foreign-object (p '(:struct struct-pair) 2)
      (setf (mem-aref p '(:struct struct-pair) 0) '(1 . 2)
            (mem-aref p '(:struct struct-pair) 1) '(3 . 4))
      (values (mem-aref p '(:struct struct-pair) 0)
              (mem-aref p '(:struct struct-pair) 1)))
(1 . 2)
(3 . 4)