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_3_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=3ad4549fc741360a6178b9bcaf2acebe5d9e8133

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