reset @@timestamp for COM_CHANGE_USER

this also fixes tokudb.type_year failure
(in ./mtr tokudb.type_timestamp tokudb.type_year)

Author: vuvova

mysql-test/r/change_user.result

@@ -90,3 +90,14 @@ NULL
 FLUSH STATUS;
 Value of com_select did not change
 set global secure_auth=default;
+set timestamp=unix_timestamp('2010-10-10 10:10:10');
+select now();
+now()
+2010-10-10 10:10:10
+select year(now()) > 2011;
+year(now()) > 2011
+0
+change_user
+select year(now()) > 2011;
+year(now()) > 2011
+1

mysql-test/t/change_user.test

@@ -142,3 +142,10 @@ if ($after != $before){
 echo Value of com_select did not change;
 
 set global secure_auth=default;
+
+set timestamp=unix_timestamp('2010-10-10 10:10:10');
+select now();
+select year(now()) > 2011;
+--echo change_user
+--change_user
+select year(now()) > 2011;

sql/sql_class.cc

@@ -945,7 +945,6 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
   statement_id_counter= 0UL;
   // Must be reset to handle error with THD's created for init of mysqld
   lex->current_select= 0;
-  user_time.val= start_time= start_time_sec_part= 0;
   start_utime= utime_after_query= 0;
   utime_after_lock= 0L;
   progress.arena= 0;
@@ -1399,6 +1398,8 @@ void THD::init(void)
 
   mysql_mutex_unlock(&LOCK_global_system_variables);
 
+  user_time.val= start_time= start_time_sec_part= 0;
+
   server_status= SERVER_STATUS_AUTOCOMMIT;
   if (variables.sql_mode & MODE_NO_BACKSLASH_ESCAPES)
     server_status|= SERVER_STATUS_NO_BACKSLASH_ESCAPES;