The old code did set max_records to either number_of_rows
(partial_join_cardinality) or memory size (join_buffer_space_limit)
which did not make sense.

Fixed by setting max_records to number of rows that fits into
join_buffer_size.

Other things:
- Initialize buffer cache values in JOIN_CACHE constructors (safety)

Reviewer: Sergei Petrunia <sergey@mariadb.com>