pgsql: Fix pg_size_pretty() to avoid overflow for inputs close to INT64

Fix pg_size_pretty() to avoid overflow for inputs close to INT64_MAX.

The expression that tried to round the value to the nearest TB could
overflow, leading to bogus output as reported in bug #5993 from Nicola
Cossu.  This isn't likely to ever happen in the intended usage of the
function (if it could, we'd be needing to use a wider datatype instead);
but it's not hard to give the expected output, so let's do so.

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/049e8b07fa076ac20662180152f122ba59f96f9f

Modified Files
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src/backend/utils/adt/dbsize.c |    8 +++++++-
1 files changed, 7 insertions(+), 1 deletions(-)