Fix extstat collection when no stats are produced for a column

This is a mistakenly placed conditional in bf2a691e.

Reported by Justin Pryzby
Discussion: https://postgr.es/m/20171117214352.GE25796@telsasoft.com

src/backend/statistics/extended_stats.c

@@ -95,8 +95,9 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
 		 */
 		stats = lookup_var_attr_stats(onerel, stat->columns,
 									  natts, vacattrstats);
-		if (!stats && !IsAutoVacuumWorkerProcess())
+		if (!stats)
 		{
+			if (!IsAutoVacuumWorkerProcess())
 				ereport(WARNING,
 						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 						 errmsg("statistics object \"%s.%s\" could not be computed for relation \"%s.%s\"",