pgsql-server: Add a check on file size as an additional safety check

Log Message:
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Add a check on file size as an additional safety check that a WAL file
recovered from archive is not corrupt. It's not much but it will catch
one common problem, viz out-of-disk-space.
Also, force a WAL recovery scan when recovery.conf is present, even if
pg_control shows a clean shutdown. This allows recovery with a tar backup
that was taken with the postmaster shut down, as per complaint from
Mark Kirkwood.

Modified Files:
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pgsql-server/src/backend/access/transam:
xlog.c (r1.150 -> r1.151)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.150&r2=1.151)