Shift from direct usage of (char *) with STRING_WITH_LEN
to C_STRING_WITH_LEN


sql/event_db_repository.cc:
  don't use direct casting but existing C_STRING_WITH_LEN macro
  to shut up the compiler.
sql/event_scheduler.cc:
  don't use direct casting but existing C_STRING_WITH_LEN macro
  to shut up the compiler.