help in tracking down issue #63 (memory corruption).  UNIV_BTR_DEBUG
is currently enabled in univ.i.

btr_root_fseg_validate(): New function, for validating a file segment
header on a B-tree root page.

btr_root_block_get(), btr_free_but_not_root(),
btr_root_raise_and_insert(), btr_discard_only_page_on_level():
Check PAGE_BTR_SEG_LEAF and PAGE_BTR_SEG_TOP on the root page with
btr_root_fseg_validate().

btr_root_raise_and_insert(): Move the assertion
dict_index_get_page(index) == page_get_page_no(root)
inside UNIV_BTR_DEBUG.  It was previously enabled by UNIV_DEBUG.

btr_free_root(): Check PAGE_BTR_SEG_TOP on the root page with
btr_root_fseg_validate().