Before the speedc test was interrupted, 121,765 B-tree pages were written.

buf_flush_init_for_writing(): Do not compress other than B-tree pages
outside the system tablespace.  Report non-B-tree pages.

page_zip_decompress(): Clear the unused heap space on the uncompressed page,
so that the whole buffer for the uncompressed page will be initialized and
page_zip_validate() will always succeed.