Fixed crash in is_stat_table() when using hash joins.

Other usage if persistent statistics is checking 'stats_is_read' in
caller, which is why this was not noticed earlier.

Other things:
- Simplified no_stat_values_provided

sql/sql_statistics.h

@@ -445,9 +445,7 @@ public:
   */
   bool no_stat_values_provided()
   {
-    if (column_stat_nulls == no_values_provided_bitmap())
-      return true;
-    return false;
+    return (column_stat_nulls == no_values_provided_bitmap());
   }
 };