"mysql_send_long_data() API call is completely broken".

Now we are resetting some members (long_data_supplied/null_value...) of Item_param to its 
initial state after each execution of prepared statement. We also manipulating 
Item_param::maybe_null/null_value only via Item_param::set_* setters which makes code a bit 
more robust.


sql/item.cc:
  Now we are assuming that Item_param may be NULL until we know this fact exactly.
  Added non-empty implementation of Item_param::reset() method which should be used
  for restoring Item_param state after each statment execution. (We need to clear 
  long_data_supplied flag, we also clear some other Item_param members here since it
  makes code simpler.)
sql/item.h:
  Now Item_param::reset() method really does something.
sql/sql_prepare.cc:
  Now we are calling Item_param::reset() for each parameter after execution for resetting Item_param
  to initial state. So we no longer don't need Prepared_statement::long_data_flag. We also 
  set Item_param::null_value/maybe_null value in Item_param::set_* and reset() methods 
  instead of doing it explicitly in insert_params_* functions (this by the way lowers 
  probability that we will forget to update one of such functions).
tests/client_test.c:
  Added test for Bug#1664 "mysql_send_long_data() API call is broken".