BUG#49481: RBR: MyISAM and bit fields may cause slave to stop on delete: 
cant find record
      
BUG#49482: RBR: Replication may break on deletes when MyISAM tables + 
char field are used

When using MyISAM tables, despite the fact that the null bit is
set for some fields, their old value is still in the row. This
can cause the comparison of records to fail when the slave is
doing an index or range scan.

We fix this by avoiding memcmp for MyISAM tables when comparing
records. Additionally, when comparing field by field, we first
check if both fields are not null and if so, then we compare
them. If just one field is null we return failure immediately. If
both fields are null, we move on to the next field.