with only COMMENT clause. Strangely it has manifestated itself
only on two platforms. This is a fix for bug#23423 
"Syntax error for "ALTER EVENT ... COMMENT ..." on just two platforms"


sql/sql_yacc.yy:
  if COMMENT is used mark it as used.
  There was a missing setting to 1 of $$. If COMMENT
  was used in combinations with other clauses then it
  was passing.