When PRIMARY KEY is present ha_innobase::cmp_ref() uses it to compare refs.
After comparing part of key it moves pointers to compare next part.
For varchar parts pointers were moved only by length of parts, not including
bytes containig part length itself. This results in wrong comparision and
wrong number of deleted records.



sql/ha_innodb.cc:
  Fix bug #12340  ha_innobase::cmp_ref() moves pointers by wrong length.
mysql-test/t/innodb.test:
  Test case for bug #12340 ha_innobase::cmp_ref() moves pointers by wrong length.
mysql-test/r/innodb.result:
  Test case for bug #12340 ha_innobase::cmp_ref() moves pointers by wrong length.