Row-based replication crashes when replicating from pre-5.1.22 to 5.1.22
due to an uninitialized variable.


sql/log_event.cc:
  Initializing m_null_bits to null so that it can be detected that it
  was not initialized later.
mysql-test/suite/rpl/r/rpl_bug31076.result:
  New BitKeeper file ``mysql-test/suite/rpl/r/rpl_bug31076.result''
mysql-test/suite/rpl/t/rpl_bug31076.test:
  New BitKeeper file ``mysql-test/suite/rpl/t/rpl_bug31076.test''