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.

src/backend/optimizer/plan/planner.c

@@ -1536,9 +1536,11 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 								  standard_qp_callback, &qp_extra);
 
 		/*
-		 * Extract rowcount and width estimates for use below.
+		 * Extract rowcount and width estimates for use below.  If final_rel
+		 * has been proven dummy, its rows estimate will be zero; clamp it to
+		 * one to avoid zero-divide in subsequent calculations.
 		 */
-		path_rows = final_rel->rows;
+		path_rows = clamp_row_est(final_rel->rows);
 		path_width = final_rel->width;
 
 		/*