pgsql: Avoid calling PageGetSpecialPointer() on an all-zeros page.

Avoid calling PageGetSpecialPointer() on an all-zeros page.

That was otherwise harmless, but tripped the new assertion in
PageGetSpecialPointer().

Reported by Amit Langote. Backpatch to 9.5, where the assertion was added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/65c384c5abee7df6d27f98135790ea24c1b9578b

Modified Files
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src/backend/access/nbtree/nbtree.c |    6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)