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

Log Message:
-----------
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.

Modified Files:
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    pgsql/src/backend/access/transam:
        xlog.c (r1.375 -> r1.376)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.375&r2=1.376)