The slave was not able to find the correct row in the innodb
table, because the row fetched from the innodb table would not
match the before image. This happened because the (don't care)
bytes in the NULLed fields would change once the row was stored
in the storage engine (from zero to the default value). This
would make bulk memory comparison (using memcmp) to fail.
  
We fix this by taking a preventing measure and avoiding memcmp
for tables that contain nullable fields. Therefore, we protect
the slave search routine from engines that return arbitrary
values for don't care bytes (in the nulled fields). Instead, the
slave thread will only check null_bits and those fields that are
not set to NULL when comparing the before image against the
storage engine row.