by adding sleep-and-retries (max 4 times) if MASTER_POS_WAIT() returns NULL
in sync_with_master and sync_slave_with_master.
The problem showed up only today, in MySQL 5.0 in rpl_server_id2.test,
but may affect 4.x as well, so fixing it here. Note that I am also fixing
5.0 too, with the same exact patch, because I don't want to leave 5.0 broken
until the next 4.0->4.1->5.0 merge.