Fixed mdev-15162 Query with CTE hangs if assignment operator (:=) is used

If setting user variable was used in the specification of a recursive CTE
then Item_func_set_user_var::fix_fields() went into an infinite loop.

mysql-test/t/cte_recursive.test

@@ -2085,3 +2085,15 @@ eval explain extended $q;
 set standard_compliant_cte=default;
 
 drop table flights;
+
+--echo #
+--echo # MDEV-15162: Setting user variable in recursive CTE
+--echo #
+
+SET @c=1;
+
+WITH RECURSIVE cte AS
+	(SELECT 5
+	UNION
+	SELECT @c:=@c+1 FROM cte WHERE @c<3)
+SELECT * FROM cte;