Problem: When an Incident_log_event contains a bad incident number on disk,
the server crashes with an assertion.
Fix: Don't validate input with assertions. Use errors.

mysql-test/include/cleanup_fake_relay_log.inc:
  Added auxiliary file to restore things that setup_fake_relay_log.inc did.
mysql-test/include/setup_fake_relay_log.inc:
  Added auxiliary file to setup replication from an existing relay log.
mysql-test/std_data/bug40482-bin.000001:
  Binlog file for rpl.rpl_binlog_corruption
mysql-test/suite/rpl/t/rpl_binlog_corruption.test:
  New test file.
sql/log_event.cc:
  Check that the incident number is correct at the time the event is constructed.
  Do not assert it at the time it is printed.
sql/log_event.h:
  Incident_log_event::is_valid() should verify that the incident number is valid.
sql/rpl_constants.h:
  Incident numbers should be hard-coded, since they may appear in files.