Rotate_log_event::exec_event() believed that the relay log was corrupted. Fixed it
by moving the test for corruption to Start_log_event::exec_event(). 
Changed Rotate_log_event::exec_event() to not increment positions when the
event is seen in the middle of a transaction.
I did a separate commit in 4.1 (so this should not be merged to 4.0) because
code is a bit different in 4.1.
A test to see if the slave detects when the master died while writing a 
transaction to the binlog (uses a forged truncated binlog I made).