buf_buddy_free(), buf_buddy_free_low(): Document that the data must
not be pointed to by the buffer pool.  Add ut_ad(!buf_pool_contains_zip())
assertions to catch pointers to freed blocks.

Validate the zip_free[] lists #ifdef UNIV_DEBUG.

buf_buddy_relocate(): Ensure that the size of the compressed page
matches.  The buddy block can be split, and the control block can be
pointing to a smaller compressed page than the one whose relocation
is being attempted.