btr_create(), btr_root_raise_and_insert(): Write a MLOG_MEMSET record
to set FIL_PAGE_PREV,FIL_PAGE_NEXT to FIL_NULL, instead of writing
two MLOG_4BYTES records.

For ROW_FORMAT=COMPRESSED pages, we will not use MLOG_MEMSET
because we want the crash-downgrade to earlier 10.4 releases to succeed.

mlog_parse_nbytes(): Relax the too strict assertion. There is no problem
with MLOG_MEMSET records that affect the uncompressed header of
ROW_FORMAT=COMPRESSED index pages.