"wrong Relay_Log_Pos if Rotate is in the middle of a transaction in relay log"
increment 'pending' instead of 'relay_log_pos'.


mysql-test/r/rpl_trunc_binlog.result:
  result update
sql/log_event.cc:
  - corrected error message
  - in Rotate_log_event::exec_event(), if we're in a transaction (which
  can happen if the I/O slave thread reconnected while reading a
  transaction), don't increment relay_log_pos but pending instead.
  Otherwise, relay_log_pos becomes garbage (and so if slave stops at that
  moment, it will never restart).