pgsql: Properly initialize write, flush and replay locations in walsend

Properly initialize write, flush and replay locations in walsender slots

These would leak random xlog positions if a walsender used for backup would
a walsender slot previously used by a replication walsender.

In passing also fix a couple of cases where the xlog pointer is directly
compared to zero instead of using XLogRecPtrIsInvalid, noted by
Michael Paquier.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/61c7bee2196d284a19692d83b33eac7588693292

Modified Files
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src/backend/replication/walsender.c |    9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)