result": new Item_param member for use in val_str()


sql/item.cc:
  Fix for Bug#3796:
  - return str_value_ptr from val_str to prevent modifications of parameter 
  value.
sql/item.h:
  Fix for bug#3796:
  - Item::val_str() method semantics documented
  - new member of Item_param
tests/client_test.c:
  Test case for bug #3796 added. A few compile-time warnings removed.