Problem:
read_set for the field is not set because its not part of the query.
As a result, server exits trying to read the data from the buffer.

Analysis:
For the query in the bugpage, as field "b" is not part of the query, optimizer
does not set the bit for the field in read_set. This results in innodb
not fetching the data for field "b". But "b" is part of the primary key. So
optimizer tries to read the key and fails.

Solution:
prepare_for_position() called from make_join_read_info() marks the fields
in read_set, which are part of primary key, if primary key is needed to
find the row.
We now call prepare_for_position() if window functions are present.