fsp_page_create(): Always initialize the page. The logic to
avoid initialization was made redundant and should have been removed
in mysql/mysql-server@ce0a1e85e24e48b8171f767b44330da635a6ea0a
(MySQL 5.7.5).

btr_store_big_rec_extern_fields(): Remove the redundant initialization
of FIL_PAGE_PREV and FIL_PAGE_NEXT. An INIT_PAGE record will have
been written already. Only write the ROW_FORMAT=COMPRESSED page payload
from FIL_PAGE_DATA onwards. We were unnecessarily writing from
FIL_PAGE_TYPE onwards, which caused an assertion failure on recovery:

	recv_sys_t::alloc(size_t): Assertion 'len <= srv_page_size' failed

when running the following tests:

	./mtr --no-reorder innodb_zip.blob,4k innodb_zip.bug56680,4k