Item::fix_field need correct pointer on item reference to chnge it if itis need, so support of correct item address added to SP commands (BUG#5963)
some optimisation of IF/NOT IF ptomised to Pem


mysql-test/r/sp.result:
  test for bug#5963
mysql-test/t/sp.test:
  test for bug#5963
sql/item.cc:
  new method which return reference on Item for SP variables support
sql/item.h:
  comment fixed
  method added
sql/sp_head.cc:
  preparation of item made separate function
  we do not need new constant Item to check IF/IF NOT
  support of passing correct address of item for fix_fields method
sql/sp_rcontext.cc:
  support of Item address passing to fix_fields
sql/sp_rcontext.h:
  support of correct address passing to fix_fields
sql/sql_class.cc:
  support of correct item address passing to fix_field