bugfix: Item_func_min_max stored thd internally

It was used for get_datetime_value() and for thd->is_error().

But in fact, get_datetime_value() never used thd argument, because the
cache ptr argument was NULL. And thd->is_error() check was not needed
at that place at all.

mysql-test/suite/vcol/r/wrong_arena.result

@@ -44,3 +44,18 @@ Warning	1292	Incorrect datetime value: '1'
 Warning	1292	Incorrect datetime value: '1'
 Warning	1292	Incorrect datetime value: '2'
 drop table t1;
+create table t1 (a datetime,
+b datetime as (least(a,1))                    # Item_func_min_max::get_date
+);
+insert t1 (a) values ('2010-10-10 10:10:10');
+select * from t1;
+a	b
+2010-10-10 10:10:10	0000-00-00 00:00:00
+Warnings:
+Warning	1292	Incorrect datetime value: '1'
+select * from t1;
+a	b
+2010-10-10 10:10:10	0000-00-00 00:00:00
+Warnings:
+Warning	1292	Incorrect datetime value: '1'
+drop table t1;

mysql-test/suite/vcol/t/wrong_arena.test

@@ -22,3 +22,14 @@ disconnect con1;
 connection default;
 select * from t1;
 drop table t1;
+
+connect con1, localhost, root;
+create table t1 (a datetime,
+  b datetime as (least(a,1))                    # Item_func_min_max::get_date
+);
+insert t1 (a) values ('2010-10-10 10:10:10');
+select * from t1;
+disconnect con1;
+connection default;
+select * from t1;
+drop table t1;

sql/item_func.cc

@@ -2856,7 +2856,6 @@ void Item_func_min_max::fix_length_and_dec()
   decimals=0;
   max_length=0;
   maybe_null=0;
-  thd= current_thd;
   cmp_type=args[0]->result_type();
 
   for (uint i=0 ; i < arg_count ; i++)
@@ -2929,13 +2928,11 @@ bool Item_func_min_max::get_date(MYSQL_TIME *ltime, ulonglong fuzzy_date)
   {
     Item **arg= args + i;
     bool is_null;
-    longlong res= get_datetime_value(thd, &arg, 0, compare_as_dates, &is_null);
+    longlong res= get_datetime_value(0, &arg, 0, compare_as_dates, &is_null);
 
     /* Check if we need to stop (because of error or KILL) and stop the loop */
-    if (thd->is_error() || args[i]->null_value)
-    {
+    if (args[i]->null_value)
       return (null_value= 1);
-    }
 
     if (i == 0 || (res < min_max ? cmp_sign : -cmp_sign) > 0)
       min_max= res;

sql/item_func.h

@@ -1068,7 +1068,6 @@ class Item_func_min_max :public Item_func
   int cmp_sign;
   /* An item used for issuing warnings while string to DATETIME conversion. */
   Item *compare_as_dates;
-  THD *thd;
 protected:
   enum_field_types cached_field_type;
 public: