buf_flush_init_for_writing(): Improve the diagnostics and make the
condition for skipping pages accurate.

univ.i: Introduce UNIV_ZIP_DEBUG for enabling some page_zip_validate() tests.

page0zip.h, page0zip.c: Define and use page_zip_validate() in
page_zip_compress() and page_zip_write() if UNIV_ZIP_DEBUG or UNIV_DEBUG
is defined.