Fix calculation of minimum in colo_compare_tcp

GitHub's CodeQL reports a critical error which is fixed by using the MIN macro:

    Unsigned difference expression compared to zero

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Cc: qemu-stable@nongnu.org
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/colo-compare.c

@@ -412,8 +412,7 @@ static void colo_compare_tcp(CompareState *s, Connection *conn)
      * can ensure that the packet's payload is acknowledged by
      * primary and secondary.
     */
-    uint32_t min_ack = conn->pack - conn->sack > 0 ?
-                       conn->sack : conn->pack;
+    uint32_t min_ack = MIN(conn->pack, conn->sack);
 
 pri:
     if (g_queue_is_empty(&conn->primary_list)) {