pgsql: Don't assume that PageIsEmpty() returns true on an all-zeros pag

Don't assume that PageIsEmpty() returns true on an all-zeros page.

It does currently, and I don't see us changing that any time soon, but we
don't make that assumption anywhere else.

Per Tom Lane's suggestion. Backpatch to 9.2, like the previous patch that
added this assumption.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/0e98ad0915ce5a665f39707db7652c67a64b5b22

Modified Files
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src/backend/access/spgist/spgvacuum.c |    2 +-
1 file changed, 1 insertion(+), 1 deletion(-)