After further I/O on a tablespace has been stopped
(for example due to DROP TABLE or an operation that
rebuilds a table), page cleaner thread tries to
flush the pending writes for the tablespace and
releases the tablespace reference even though it was not
acquired.

fil_space_t::flush(): Don't release the tablespace when it is
being stopped and closed

Thanks to Marko Mäkelä for suggesting this patch.