fsp_alloc_seg_inode_page(): Ever since
commit 3926673c
all newly allocated pages are zero-initialized.
Assert that this is the case for the FSEG_ID fields.
(Side note: before that fix, other parts of the pages
could contain nonzero garbage.)

btr_store_big_rec_extern_fields(): Remove the redundant initialization
of the most significant 32 bits of BTR_EXTERN_LEN. InnoDB never supported
BLOBs that are longer than 4GiB. In fact, dtuple_convert_big_rec()
would write emit an error message if a clustered index record tuple would
exceed 1,000,000,000 bytes in length.