pgsql: Before removing backup_label and irrevocably changing pg_control

Before removing backup_label and irrevocably changing pg_control file, check
that WAL file containing the checkpoint redo-location can be found. This
avoids making the cluster irrecoverable if the redo location is in an earlie
WAL file than the checkpoint record.

Report, analysis and patch by Jeff Davis, with small changes by me.

Branch
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REL8_2_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=49b5aba40e0b0adb1caaea81c88dd0e54a7d3890

Modified Files
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src/backend/access/transam/xlog.c |   19 +++++++++++++++++--
1 files changed, 17 insertions(+), 2 deletions(-)