This fixes a problem in replication where RESET SLAVE could crash a running slave.
mysql-test/r/rpl_failsafe.result:
Added missing drop
mysql-test/r/rpl_log_pos.result:
Added slave stop before change master
mysql-test/t/rpl_log_pos.test:
Added slave stop before change master
sql/sql_parse.cc:
reset_slave() sends it's own errors
sql/sql_repl.cc:
reset_slave() sends it own error.
RESET SLAVE and CHANGE MASTER will now give an error if slave is running.
This fixes a problem in replication where RESET SLAVE could crash a running slave.
