Fixed quick_sort when array is length 0 or 1

[Louis-Vincent]

Fixed a bug where quick_sort could access out of bound element when to argument smaller or equal to 0.

Before

var xs = [1] default_comparator.quick_sort(xs) # assertion failed

After

var xs = [1] default_comparator.quick_sort(xs) assert xs == [1] # true

Signed-off-by: Louis-Vincent Boudreault lv.boudreault95@gmail.com

[Delja]

Good catch!