pgsql: Guard against possible memory allocation botch in batchmemtuples

Guard against possible memory allocation botch in batchmemtuples().

Negative availMemLessRefund would be problematic.  It's not entirely
clear whether the case can be hit in the code as it stands, but this
seems like good future-proofing in any case.  While we're at it,
insist that the value be not merely positive but not tiny, so as to
avoid doing a lot of repalloc work for little gain.

Peter Geoghegan

Discussion: <CAM3SWZRVkuUB68DbAkgw=532gW0f+fofKueAMsY7hVYi68MuYQ@mail.gmail.com>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f032722f86a709277d44a317a3bc8acd74861a72

Modified Files
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src/backend/utils/sort/tuplesort.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)