pgsql: Fix an uninitialized access in hash_xlog_squeeze_page().

Fix an uninitialized access in hash_xlog_squeeze_page().

Commit 861f86beea changed hash_xlog_squeeze_page() to start reading
the write buffer conditionally but forgot to initialize it leading to an
uninitialized access.

Reported-by: Alexander Lakhin
Author: Hayato Kuroda
Reviewed-by: Alexander Lakhin, Amit Kapila
Discussion: http://postgr.es/m/62ed1a9f-746a-8e86-904b-51b9b806a1d9@gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/f66fcc5cd616da8d81a2d2f9b80a9f080796f4b6

Modified Files
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src/backend/access/hash/hash_xlog.c      |  2 +-
src/test/regress/expected/hash_index.out | 14 ++++++++++++++
src/test/regress/sql/hash_index.sql      | 17 +++++++++++++++++
3 files changed, 32 insertions(+), 1 deletion(-)