Avoid misinterpreting GiST pages in pageinspect.
GistPageSetDeleted() sets pd_lower when deleting a page, and sets the
page contents to a GISTDeletedPageContents. Avoid treating deleted GiST
pages as regular slotted pages within pageinspect.
Oversight in commit
756ab291.
Author: Andrey Borodin <x4mmm@yandex-team.ru>