Memory leak in locally evalutated expressions during SP execution fixed by
  reusing allocated item slots when possible.
  Note: No test case added, since the test is a stress test that tries to make
  the machine to run out of memory.


sql/item.cc:
  Make it possible to reuse allocated item slots (for use in SP execution).
sql/item.h:
  Make it possible to reuse allocated item slots (for use in SP execution).
sql/sp_head.cc:
  Reuse allocated item slots for expression evalutation during SP execution.
sql/sp_rcontext.cc:
  Updated sp_eval_func_item() call, and prevent item reuse in reused frames (for handlers).