When there is a secondary index on a column prefix of an externally
stored column and an entry in the secondary index is shorter than the
reserved prefix length, it should mean that the secondary index entry
is holding the complete column value. When comparing this secondary
index column value to the column in the clustered index row, we must
compare the entire prefix that was fetched from the clustered
index. The bug was that we would just compare that the column in the
clustered index starts with the value found in the secondary index
column.

This bug affects only the InnoDB Barracuda formats (ROW_FORMAT=DYNAMIC
and ROW_FORMAT=COMPRESSED), in which columns that are stored off-page
in the clustered index do not contain any prefix in the clustered
index record.

row_sel_sec_rec_is_for_blob(): Add the parameter prefix_len, for
ifield->prefix_len. Add some assertions.

Sorry, I did not manage to produce a test case. This patch does
produce correct results on the data set that Michael isolated on our
test machine. That was with the purge and background rollback
suspended, because they would make the bug go away.

rb:760 approved by Sunny Bains