pgsql: Protect GIST logic that assumes penalty values can't be negative

Protect GIST logic that assumes penalty values can't be negative.

Apparently sane-looking penalty code might return small negative values,
for example because of roundoff error.  This will confuse places like
gistchoose().  Prevent problems by clamping negative penalty values to
zero.  (Just to be really sure, I also made it force NaNs to zero.)
Back-patch to all supported branches.

Alexander Korotkov

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ca76a3956c80f97eb4ea0cc8fc2a54435f6a573b

Modified Files
--------------
doc/src/sgml/gist.sgml             |    2 ++
src/backend/access/gist/gistutil.c |   14 +++++++++++---
2 files changed, 13 insertions(+), 3 deletions(-)