btr_store_big_rec_extern_fields(): Assert that page_zip is non-NULL
if and only if dict_table_is_zip() holds.

btr_free_externally_stored_field(): Observe dict_table_is_zip().
Allow page_zip==NULL even if dict_table_is_zip().  Remove the
related TODO comment in row_purge_upd_exist_or_extern().

page_zip_available(): uncompressed_size already includes
PAGE_ZIP_DIR_SLOT_SIZE.

page_zip_decompress(): Remove bogus assertion d_stream.next_out == last.
Do not subtract BTR_EXTERN_FIELD_REF_SIZE from d_stream.avail_in when
decompressing records, because the records may be deleted later in
page_zip_apply_log(), and no BLOB pointers are allocated for deleted
records.