array.c: check for negative shift size.

src/array.c

@@ -591,6 +591,7 @@ mrb_ary_shift_m(mrb_state *mrb, mrb_value self)
   };
   ary_modify_check(mrb, a);
   if (len == 0 || n == 0) return mrb_ary_new(mrb);
+  if (n < 0) mrb_raise(mrb, E_ARGUMENT_ERROR, "negative array shift");
   if (n > len) n = len;
   val = mrb_ary_new_from_values(mrb, n, ARY_PTR(a));
   if (ARY_SHARED_P(a)) {