Guard against zero vardata.rel->tuples in estimate_hash_bucketsize().

If the referenced rel was proven empty, we'd compute 0/0 here, which
results in the function returning NaN.  That's a bit more serious
than the other zero-divide case.  Still, it only seems to be possible
in HEAD, so no back-patch.

Per report from Piotr Stefaniak.  I looked through the rest of selfuncs.c
and found no other likely trouble spots.

src/backend/utils/adt/selfuncs.c

@@ -3540,7 +3540,7 @@ estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets)
 	 * XXX Possibly better way, but much more expensive: multiply by
 	 * selectivity of rel's restriction clauses that mention the target Var.
 	 */
-	if (vardata.rel)
+	if (vardata.rel && vardata.rel->tuples > 0)
 	{
 		ndistinct *= vardata.rel->rows / vardata.rel->tuples;
 		ndistinct = clamp_row_est(ndistinct);