For a correlated subquery filesort is executed multiple times.
During each execution, sortlength() computed total sort key length in
Sort_keys::sort_length, without resetting it first.

Eventually Sort_keys::sort_length got larger than @@sort_buffer_size, which
caused filesort() to be aborted with error.

Fixed by making sortlength() to compute lengths only during the first
invocation. Subsequent invocations return pre-computed values.