Field_bit::set_default() did not check the bit_len, hence used the undefined
bit_ptr, causing a crash. The patch adds a check that bit_len > 0 before
following the bit_ptr.


mysql-test/extra/rpl_tests/rpl_row_tabledefs.test:
  Doing select using ORDER BY to prevent table-internal order from
  affecting the result.
mysql-test/r/rpl_row_tabledefs_2myisam.result:
  Result change
mysql-test/r/rpl_row_tabledefs_3innodb.result:
  Result change
sql/field.cc:
  Checking bit_len before following the bit_ptr, since bit_ptr has no
  sensible value in the case that bit_len == 0.
sql/field.h:
  Field_bit::set_default() used the bit_ptr, but it is undefined,
  hence causing a crash.  In reality, the hierarchy order is not correct
  so added a TODO comment about refactoring.
sql/log_event.cc:
  Code was manipulating bits for a FIELD_TYPE_BIT field without checking
  if the bit_len was > 0, hence using an undefined bit_ptr when the
  class was actually a Field_bit_as_char.
mysql-test/t/rpl_row_tabledefs_3innodb-slave.opt:
  New BitKeeper file ``mysql-test/t/rpl_row_tabledefs_3innodb-slave.opt''