A page with multiple records or deleted records still does not compress
or decompress properly.

buf_flush_init_for_writing(): Initialize block->page_zip properly so that all
assertions in page0zip can be enabled.

page_zip_decompress(): Note that corrupt data should not lead to assertions.

page_zip_dir_set(): Correct the interface.  Fix off-by-one error.
page_zip_dir_get(): Fix off-by-one error.

page0zip.c: Replace n_heap with n_dense and add comments about
the infimum and supremum records whenever we subtract 2 from heap_no.
Fix some programming errors.