Fixed BUG#3368: Stored procedure crash if group_concat(variable).

mysql-test/r/sp.result:
  New test case for BUG#3368.
mysql-test/t/sp.test:
  New test case for BUG#3368.
sql/item.h:
  Corrected const_item() for item_splocal. (Confusing semantics here, but this
  makes group_concat() behave as expected, instead of crashing, and does not
  appear to affect anything else.)

mysql-test/r/sp.result

@@ -1372,6 +1372,21 @@ select 1+2|
 1+2
 3
 drop procedure bug3843|
+drop table if exists t3|
+create table t3 ( s1 char(10) )|
+insert into t3 values ('a'), ('b')|
+create procedure bug3368(v char(10))
+begin
+select group_concat(v) from t3;
+end|
+call bug3368('x')|
+group_concat(v)
+x,x
+call bug3368('yz')|
+group_concat(v)
+yz,yz
+drop procedure bug3368|
+drop table t3|
 drop table if exists fac|
 create table fac (n int unsigned not null primary key, f bigint unsigned)|
 create procedure ifac(n int unsigned)

mysql-test/t/sp.test

@@ -1573,6 +1573,25 @@ select 1+2|
 
 drop procedure bug3843|
 
+#
+# BUG#3368
+#
+--disable_warnings
+drop table if exists t3|
+--enable_warnings
+create table t3 ( s1 char(10) )|
+insert into t3 values ('a'), ('b')|
+
+create procedure bug3368(v char(10))
+begin
+  select group_concat(v) from t3;
+end|
+
+call bug3368('x')|
+call bug3368('yz')|
+drop procedure bug3368|
+drop table t3|
+
 
 #
 # Some "real" examples

sql/item.h

@@ -332,7 +332,7 @@ public:
 
   inline bool const_item() const
   {
-    return FALSE;
+    return TRUE;
   }
 
   inline int save_in_field(Field *field, bool no_conversions)