pgsql: Fix calculation of how many segments to retain for wal_keep_segm

Fix calculation of how many segments to retain for wal_keep_segments.

KeepLogSeg function was broken when we switched to use a 64-bit int for the
segment number.

Per report from Jeff Janes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/594041311c8263832258d35632aa3eebf9371fd1

Modified Files
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src/backend/access/transam/xlog.c |    8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)