Debug measure: trash unused buffer space

sql/multi_range_read.h

@@ -113,7 +113,7 @@ public:
     start= start_arg;
     end= end_arg;
     direction= direction_arg;
-  //  TRASH(start, end - start);
+    TRASH(start, end - start);
     reset_for_writing();
   }
   
@@ -153,6 +153,8 @@ public:
       - it is adjacent to buffer space we're using
       - it is on the end towards which we grow.
     */
+    DBUG_ASSERT(unused_end > unused_start);
+    TRASH(unused_start, unused_end - unused_start);
     if (direction == 1 && end == unused_start)
     {
       end= unused_end;