Do not adjust the remainder of computer_div_mod if it is zero.

This fixes (-2) mod 2 being evaluated as nonzero.

src/util/bigInt.ml

@@ -44,13 +44,13 @@ let euclidean_div x y = fst (euclidean_div_mod x y)
 let euclidean_mod x y = snd (euclidean_div_mod x y)
 
 let computer_div_mod x y =
-  let q,r = quomod_big_int x y in
+  let (q,r) as qr = quomod_big_int x y in
   (* we have x = q*y + r with 0 <= r < |y| *)
-  if sign x < 0 then
+  if sign x >= 0 || sign r = 0 then qr
+  else
     if sign y < 0
     then (pred q, add r y)
     else (succ q, sub r y)
-  else (q,r)
 
 let computer_div x y = fst (computer_div_mod x y)