Fixed where the slave code would try to update the Lex->sphead which is NULL on an "alter table" commands.


mysql-test/r/rpl_events.result:
  test that "alter event" replicates without crashing the slave
mysql-test/t/rpl_events.test:
  test that "alter event" replicates without crashing the slave
sql/sql_parse.cc:
  Added a check for lex->spd, which isn't set on an "alter event" command