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.