The server crashes when receiving a COM_BINLOG_DUMP command with a position of 0 or
larger than the file size.
The execution proceeds to an error block having the last read replication coordinates 
pointer be NULL and its dereferencing crashed the server.

Fixed with making "public" previously used only for heartbeat coordinates.


mysql-test/extra/rpl_tests/rpl_start_stop_slave.test:
  regression test for bug#3593869-64035 is added.
mysql-test/suite/rpl/r/rpl_cant_read_event_incident.result:
  results updated (error mess format is changed).
mysql-test/suite/rpl/r/rpl_log_pos.result:
  results updated (error mess format is changed).
mysql-test/suite/rpl/r/rpl_manual_change_index_file.result:
  results updated (error mess format is changed).
mysql-test/suite/rpl/r/rpl_packet.result:
  results updated (error mess format is changed).
mysql-test/suite/rpl/r/rpl_stm_start_stop_slave.result:
  results updated (error mess format is changed).
mysql-test/suite/rpl/t/rpl_stm_start_stop_slave.test:
  Slave is stopped by bug#3593869-64035 tests so 
  -let $rpl_only_running_threads= 1 is set prior to rpl_end.
sql/share/errmsg-utf8.txt:
  Increasing the max length of explanatory message to 512.
sql/sql_repl.cc:
  Making `coord' to carry the last read from binlog event coordinates
  regardless of heartbeat.
  Renaming, small cleanup and simplifying the code after if (coord) becomes unnecessary.
  Adding yet another 3rd pair of coordinates - the starting replication - 
  into error text.