Fix to make it compiling with valgrind.

sql/opt_range.cc

@@ -13324,7 +13324,7 @@ int QUICK_GROUP_MIN_MAX_SELECT::next_min()
       }
       else if (result == HA_ERR_KEY_NOT_FOUND || result == HA_ERR_END_OF_FILE)
         result= 0; /* There is a result in any case. */
-      my_afree(tmp_key_buff);
+      my_afree(key_buf);
     }
   }