pgsql: Fix STOP WAL LOCATION in backup history files no to return the

Log Message:
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Fix STOP WAL LOCATION in backup history files no to return the next
segment of XLOG_BACKUP_END record even if the the record is placed
at a segment boundary. Furthermore the previous implementation could
return nonexistent segment file name when the boundary is in segments
that has "FE" suffix; We never use segments with "FF" suffix.

Backpatch to 8.0, where hot backup was introduced.

Reported by Fujii Masao.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/access/transam:
xlog.c (r1.258.2.7 -> r1.258.2.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.258.2.7&r2=1.258.2.8)