buf_block_align() on a non-file page frame that was created in
btr_cur_pessimistic_insert(), to see if a record fits on a compressed
page by itself.  These assertions caused an assertion failure in
buf_block_align() in innodb_bug36172.test.

page_zip_write_rec(), page_zip_write_header(): Remove the assertion
that calls buf_frame_get_page_zip().