Diskless replication: don't send "\n" pings to slaves.

This is useful for normal replication in order to refresh the slave
when we are persisting on disk, but for diskless replication the
child is already receiving data while in WAIT_BGSAVE_END state.

src/replication.c

@@ -1785,7 +1785,9 @@ void replicationCron(void) {
             redisClient *slave = ln->value;
 
             if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START ||
-                slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) {
+                (slave->replstate == REDIS_REPL_WAIT_BGSAVE_END &&
+                 server.rdb_child_type != REDIS_RDB_CHILD_TYPE_SOCKET))
+            {
                 if (write(slave->fd, "\n", 1) == -1) {
                     /* Don't worry, it's just a ping. */
                 }