core: avoid NULL derefrence in grub_divmod64s

It can be called with NULL for third argument.  grub_divmod32* for
now are called only from within wrappers, so skip check.

Reported-By: Michael Zimmermann <sigmaepsilon92@gmail.com>

grub-core/lib/division.c

@@ -50,7 +50,8 @@ grub_divmod64s (grub_int64_t n,
       q = -q;
     }
   /* Now: n = d * q + r  */
-  *ro = r;
+  if (ro)
+    *ro = r;
   return q;
 }