Fixed the bug mdev12992.

When the SELECT query from a trigger that used a subquery
in its SELECT list was prepared the counter select_n_having_items
was incremented in the constructor Item::Item(THD *thd).
As a result each invocation of the trigger required more and more
memory for the ref_pointer_array for this SELECT.
Made sure that the counter st_select_lex::select_n_having_items
would be incremented only at the first execution of such trigger.

mysql-test/r/trigger.result

@@ -2368,3 +2368,14 @@ tr1	1	2016-01-01 10:10:10.33
 tr2	2	2016-01-01 10:10:10.99
 drop table t1;
 set time_zone= @@global.time_zone;
+#
+# MDEV-12992: Increasing memory consumption
+with each invocation of trigger
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (b INT);
+CREATE TRIGGER tr 
+AFTER UPDATE ON t1 FOR EACH ROW SELECT (SELECT b FROM t2) INTO @x;
+# Running 20000 queries
+DROP TABLE t1,t2;

mysql-test/t/trigger.test

@@ -2676,3 +2676,27 @@ select trigger_name, action_order, created from information_schema.triggers
   where event_object_table = 't1' and trigger_schema='test';
 drop table t1;
 set time_zone= @@global.time_zone;
+
+--echo #
+--echo # MDEV-12992: Increasing memory consumption
+--echo               with each invocation of trigger
+--echo #
+
+--let $n= 20000
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (b INT);
+CREATE TRIGGER tr 
+  AFTER UPDATE ON t1 FOR EACH ROW SELECT (SELECT b FROM t2) INTO @x;
+
+--disable_query_log
+--echo # Running $n queries
+while ($n)
+{
+    UPDATE t1 SET a = 2;
+    --dec $n
+}
+--enable_query_log
+
+DROP TABLE t1,t2;

sql/item.cc

@@ -490,7 +490,8 @@ Item::Item(THD *thd):
     command => we should check thd->lex->current_select on zero (thd->lex
     can be uninitialised)
   */
-  if (thd->lex->current_select)
+  if (thd->lex->current_select &&
+      thd->stmt_arena->is_stmt_prepare_or_first_sp_execute())
   {
     enum_parsing_place place= 
       thd->lex->current_select->parsing_place;