pgsql: Clamp adjusted ndistinct to positive integer in estimate_hash_bu

Clamp adjusted ndistinct to positive integer in estimate_hash_bucketsize().

This avoids a possible divide-by-zero in the following calculation,
and rounding the number to an integer seems like saner behavior anyway.
Assuming IEEE math, the division would yield +Infinity which would get
replaced by 1.0 at the bottom of the function, so nothing really
interesting would ensue; but avoiding divide-by-zero seems like a
good idea on general principles.

Per report from Piotr Stefaniak.  No back-patch since this seems
mostly cosmetic.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/fa09f8935156533584b4e215bdf70ec1ff968dad

Modified Files
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src/backend/utils/adt/selfuncs.c | 3 +++
1 file changed, 3 insertions(+)