row_sel_sec_rec_is_for_clust_rec(): If the field in the
clustered index record stored off page, always fetch it,
also when the secondary index field has been built on the
entire column. This was broken ever since the InnoDB Plugin
for MySQL Server 5.1 introduced ROW_FORMAT=DYNAMIC and
ROW_FORMAT=COMPRESSED for InnoDB tables. That code was first
introduced in this tree in
commit 3945d5e5.

For the original ROW_FORMAT=REDUNDANT and the MySQL 5.0.3
ROW_FORMAT=COMPRESSED, there was no problem, because for
those tables we always stored at least a 768-byte prefix of
each column in the clustered index record.

row_sel_sec_rec_is_for_blob(): Allow prefix_len==0 for matching
the full column.