pgsql: Don't call PageGetSpecialPointer() on page until it's been initi

Don't call PageGetSpecialPointer() on page until it's been initialized.

After calling XLogInitBufferForRedo(), the page might be all-zeros if it was
not in page cache already. btree_xlog_unlink_page initialized the page
correctly, but it called PageGetSpecialPointer before initializing it, which
would lead to a corrupt page at WAL replay, if the unlinked page is not in
page cache.

Backpatch to 9.4, the bug came with the rewrite of B-tree page deletion.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/fdf28853ae6a397497b79fea69f89f4f7b9aa991

Modified Files
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src/backend/access/nbtree/nbtxlog.c |    3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)