Bug#59149 valgrind warnings with "like .. escape .." function

      
Problem: when processing a query like:
  SELECT '' LIKE '1' ESCAPE COUNT(1);
escape_item->val_str() was never executed and the "escape" class member
stayed initialized, which led to valgrind uninitialized memory error.
      
Note, a query with some tables in "FROM" clause
returns ER_WRONG_ARGUMENTS in the same situation:

   SELECT '' LIKE '1' ESCAPE COUNT(1) FROM t1;
   ERROR 1210 (HY000): Incorrect arguments to ESCAPE

Fix: disallowing using aggregate functions in ESCAPE clause,
even if there are no tables used. There is no much use of that anyway.

mysql-test/r/func_like.result

@@ -182,4 +182,9 @@ INSERT INTO t2 VALUES (1), (2), (3);
 SELECT 1 FROM t2 JOIN t1 ON 1 LIKE a GROUP BY a;
 1
 DROP TABLE t1, t2;
+#
+# Bug#59149 valgrind warnings with "like .. escape .." function
+#
+SELECT '' LIKE '1' ESCAPE COUNT(1);
+ERROR HY000: Incorrect arguments to ESCAPE
 End of 5.1 tests

mysql-test/t/func_like.test

@@ -126,5 +126,10 @@ INSERT INTO t2 VALUES (1), (2), (3);
 SELECT 1 FROM t2 JOIN t1 ON 1 LIKE a GROUP BY a;
 DROP TABLE t1, t2;
 
+--echo #
+--echo # Bug#59149 valgrind warnings with "like .. escape .." function
+--echo #
+--error ER_WRONG_ARGUMENTS
+SELECT '' LIKE '1' ESCAPE COUNT(1);
 
 --echo End of 5.1 tests

sql/item_sum.h

@@ -339,6 +339,7 @@ public:
     forced_const= TRUE; 
   }
   virtual bool const_item() const { return forced_const; }
+  virtual bool const_during_execution() const { return false; }
   virtual void print(String *str, enum_query_type query_type);
   void fix_num_length_and_dec();