Frame bounds using FOLLOWING or PRECEDING can have 0 as cardinal value

This makes them behave exactly like CURRENT ROW. Standard specifies
unsigned integer, which includes the value 0.

Expand the win_min_max test to include this kind of frame definitions.

sql/sql_window.cc

@@ -1688,7 +1688,6 @@ class Frame_n_rows_following : public Frame_cursor
     is_top_bound(is_top_bound_arg), n_rows(n_rows_arg),
     cursor(thd, partition_list)
   {
-    DBUG_ASSERT(n_rows > 0);
   }
 
   void init(READ_RECORD *info)