Variable initialization

PHP warning when not feed is iddle
https://github.com/FreshRSS/FreshRSS/issues/1227#issuecomment-242702464

app/Models/StatsDAO.php

@@ -168,6 +168,7 @@ SQL;
 		$stm->execute();
 		$res = $stm->fetchAll(PDO::FETCH_NAMED);
 
+		$repartition = array();
 		foreach ($res as $value) {
 			$repartition[(int) $value['period']] = (int) $value['count'];
 		}