Fix for prepared statements:

added setting of 'neg' in Item_param::set_time() (looks like the only forgotten
member).
It's the second place I find where 'neg' was forgotten.
The symptom was unexpected negative times in the binary log when running
tests/client_test.c (test_date() in fact):
# at 43009
#040210 15:46:42 server id 1  log_pos 43009     Query   thread_id=1     exec_time=0     error_code=0
SET TIMESTAMP=1076424402;
INSERT INTO test_date VALUES('2000-01-10 11:16:20','-11:16:20' etc

sql/item.cc

@@ -568,6 +568,8 @@ void Item_param::set_time(TIME *tm, timestamp_type type)
 
   ltime.second_part= tm->second_part;
 
+  ltime.neg= tm->neg;
+
   ltime.time_type= type;
   
   item_is_time= true;