buf_page_io_complete(): On FIL_PAGE_TYPE_ZBLOB (compressed BLOB pages),
read the space_id from a different location.

page_zip_compress(), page_zip_write_rec(), page_zip_write_blob_ptr():
Replace page_simple_validate_new() with page_validate().

page_zip_clear_rec(): When running out of log space, do not attempt to
recompress the page, because the directory slots might be unbalanced and
the page_validate() assertion in page_zip_compress() would fail.
Instead, clear the BLOB pointers of the deleted record on the
uncompressed page, so that page_zip_validate() will succeed.

page_zip_validate(): Remove the comment about page_zip_clear_rec().
A mismatch always indicates a serious inconsistency.