fix bigint mod with negative numbers

Author: moritz

3rdparty/libtommath/bn_mp_mod.c

@@ -31,11 +31,11 @@ mp_mod (mp_int * a, mp_int * b, mp_int * c)
     return res;
   }
 
-  if (t.sign != b->sign) {
-    res = mp_add (b, &t, c);
-  } else {
+  if (mp_iszero(&t) || t.sign == b->sign) {
     res = MP_OKAY;
     mp_exch (&t, c);
+  } else {
+    res = mp_add (b, &t, c);
   }
 
   mp_clear (&t);