pgsql: Avoid some zero-divide hazards in the planner.

Avoid some zero-divide hazards in the planner.

Although I think on all modern machines floating division by zero
results in Infinity not SIGFPE, we still don't want infinities
running around in the planner's costing estimates; too much risk
of that leading to insane behavior.

grouping_planner() failed to consider the possibility that final_rel
might be known dummy and hence have zero rowcount. (I wonder if it
would be better to set a rows estimate of 1 for dummy relations?
But at least in the back branches, changing this convention seems
like a bad idea, so I'll leave that for another day.)

Make certain that get_variable_numdistinct() produces a nonzero result.
The case that can be shown to be broken is with stadistinct < 0.0 and
small ntuples; we did not prevent the result from rounding to zero.
For good luck I applied clamp_row_est() to all the nonconstant return
values.

In ExecChooseHashTableSize(), Assert that we compute positive nbuckets
and nbatch. I know of no reason to think this isn't the case, but it
seems like a good safety check.

Per reports from Piotr Stefaniak. Back-patch to all active branches.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/3b4a9dbfa5339405f7f78639b83bfb563603cfe3

Modified Files
--------------
src/backend/executor/nodeHash.c | 3 +++
src/backend/optimizer/plan/planner.c | 6 ++++--
src/backend/utils/adt/selfuncs.c | 10 +++++-----
3 files changed, 12 insertions(+), 7 deletions(-)