There is a potential race when we stop the slave. It may take some time for
the master to detect that the slave connection is closed (eg. if scheduling
delays the TCP RSET packet or whatever). Since we inject only a single corrupt
binlog event, we may be unfortunate enough to inject it on the wrong
connection, to a slave io thread that's already stopped.

Fix by waiting for the old dump thread on the master to go away before
injecting the corrupt event.