pgsql: pg_stat_replication.sync_state was displayed incorrectly at page

pg_stat_replication.sync_state was displayed incorrectly at page boundary.

XLogRecPtrIsInvalid() only checks the xrecoff field, which is correct when
checking if a WAL record could legally begin at the given position, but WAL
sending can legally be paused at a page boundary, in which case xrecoff is
0. Use XLByteEQ(..., InvalidXLogRecPtr) instead, which checks that both
xlogid and xrecoff are 0.

9.3 doesn't have this problem because XLogRecPtr is now a single 64-bit
integer, so XLogRecPtrIsInvalid() does the right thing. Apply to 9.2, and
9.1 where pg_stat_replication view was introduced.

Kyotaro HORIGUCHI, reviewed by Fujii Masao.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3f7b04d6f694b447c17999ef0aed406e658de82f

Modified Files
--------------
src/backend/replication/syncrep.c | 5 +++--
src/backend/replication/walsender.c | 4 ++--
2 files changed, 5 insertions(+), 4 deletions(-)