backport of bug #54476 fix from 5.1-bugteam to 5.0-bugteam.

Original revid: alexey.kopytov@sun.com-20100723115254-jjwmhq97b9wl932l

 > Bug #54476: crash when group_concat and 'with rollup' in
 >                      prepared statements
 >
 > Using GROUP_CONCAT() together with the WITH ROLLUP modifier
 > could crash the server.
 >
 > The reason was a combination of several facts:
 >
 > 1. The Item_func_group_concat class stores pointers to ORDER
 > objects representing the columns in the ORDER BY clause of
 > GROUP_CONCAT().
 >
 > 2. find_order_in_list() called from
 > Item_func_group_concat::setup() modifies the ORDER objects so
 > that their 'item' member points to the arguments list
 > allocated in the Item_func_group_concat constructor.
 >
 > 3. In some cases (e.g. in JOIN::rollup_make_fields) a copy of
 > the original Item_func_group_concat object could be created by
 > using the Item_func_group_concat::Item_func_group_concat(THD
 > *thd, Item_func_group_concat *item) copy constructor. The
 > latter essentially creates a shallow copy of the source
 > object. Memory for the arguments array is allocated on
 > thd->mem_root, but the pointers for arguments and ORDER are
 > copied verbatim.
 >
 > What happens in the test case is that when executing the query
 > for the first time, after a copy of the original
 > Item_func_group_concat object has been created by
 > JOIN::rollup_make_fields(), find_order_in_list() is called for
 > this new object. It then resolves ORDER BY by modifying the
 > ORDER objects so that they point to elements of the arguments
 > array which is local to the cloned object. When thd->mem_root
 > is freed upon completing the execution, pointers in the ORDER
 > objects become invalid. Those ORDER objects, however, are also
 > shared with the original Item_func_group_concat object which is
 > preserved between executions of a prepared statement. So the
 > first call to find_order_in_list() for the original object on
 > the second execution tries to dereference an invalid pointer.
 >
 > The solution is to create copies of the ORDER objects when
 > copying Item_func_group_concat to not leave any stale pointers
 > in other instances with different lifecycles.


mysql-test/r/func_gconcat.result:
  Test case for bug #54476.
mysql-test/t/func_gconcat.test:
  Test case for bug #54476.
sql/item_sum.cc:
  Copy the ORDER objects pointed to by the elements of the
  'order' array in the copy constructor of
  Item_func_group_concat.
sql/table.h:
  Removed the unused 'item_copy' member of the ORDER class.

sql/table.h

@@ -31,7 +31,6 @@ typedef struct st_order {
   struct st_order *next;
   Item	 **item;			/* Point at item in select fields */
   Item	 *item_ptr;			/* Storage for initial item */
-  Item   **item_copy;			/* For SPs; the original item ptr */
   int    counter;                       /* position in SELECT list, correct
                                            only if counter_used is true*/
   bool	 asc;				/* true if ascending */