pgsql: Avoid somewhat-theoretical overflow risks in RecordIsValid().

Avoid somewhat-theoretical overflow risks in RecordIsValid().

This improves on commit 51fed14d73ed3acd2282b531fb1396877e44e86a by
eliminating the assumption that we can form <some pointer value> +
<some offset> without overflow. The entire point of those tests is that
we don't trust the offset value, so coding them in a way that could wrap
around if the buffer happens to be near the top of memory doesn't seem
sound. Instead, track the remaining space as a size_t variable and
compare offsets against that.

Also, improve comment about why we need the extra early check on
xl_tot_len.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/10685ec082181b285a48d982b4c3463fd80ee5ae

Modified Files
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src/backend/access/transam/xlog.c | 26 +++++++++++++++++---------
1 files changed, 17 insertions(+), 9 deletions(-)