Expression c++ is not recognized correctly

[YaoHouyou]

version: 351acdf, b6fc4e1, e944cda

Testcase:

1. var foo = function () {
2. 	var c = 0;
3. 	var p1 = this[c];
4. 	print(p1);
5. 	var p2 = this[c++];
6. 	print(p2);
7. };
8. foo();

Command:

./jerryscript/build/bin/jerry testcase.js

Result:

undefined
Exception backtrace:
  0: 20190612/373821.js:5
  1: 20190612/373821.js:9
Script Error: ReferenceError: Undefined reference.

Comment:

Both variable p1 and variable p2 should be undefined, but jerry throws an exception on line 5. Other more, other engines do not throw exception on line 5, such as v8, spiderMonkey, chakra, javascriptCore.