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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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=3d7905d21a45ae5790ad3ae5487d56de5cf39110

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