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.
  Second attempt, now tested with debug build, valgrind build, max (optimized)
  build, with and without --debug, --vagrind and --ps-protocol.
  Errors in trigger and view test with --debug in debug build where present
  before this patch, and likewise for valgrind warnings for view test in
  valgrind build with --ps-protocol.


sql/item.cc:
  Init rsize in Item (for SP item reusal).
sql/item.h:
  Addes special new operator for reuse of Items, for SP internal use only.
sql/sp_head.cc:
  Reuse items assigned internally in SPs when possible.
sql/sp_rcontext.cc:
  Reuse items assigned internally in SPs when possible.
  Moved the local variable assignment here (from sp_head) to avoid
  duplicated code.
sql/sp_rcontext.h:
  New arg to sp_rcontext::set_item_eval() (and some coding style).
sql/sql_class.cc:
  Adjusted call to new set_item_eval().