Make sure to cleanup the items for a cursor query after each open, otherwise
it's done too late, after the run-time mem_root is freed.


mysql-test/r/sp.result:
  New test case for BUG#11529.
mysql-test/t/sp.test:
  New test case for BUG#11529.
sql/sp_head.cc:
  Add a back pointer from a sp_cursor to its cpush instruction, and use it to set
  the arena and cleanup the items for the cursor's query when opening it.
sql/sp_rcontext.cc:
  Store pointer in sp_cursor to its cpush instruction.
sql/sp_rcontext.h:
  Store pointer in sp_cursor to its cpush instruction.