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.