page_zip_compress_write_log(): Do not invoke page_zip_validate(), since
this is a static function and the callers already invoke page_zip_validate().

page_zip_decompress_low(): Clear the page #ifdef UNIV_ZIP_DEBUG.
Optimizations: Remove local variable "last".  Do not clear the BLOB
pointer until the modification log has been applied.  (Since r690,
it will be cleared after applying the modification log.)
Avoid calling rec_get_offsets() a second time if there are no
uncompressed columns to be restored.