update memory allocate check

[aronlt]

If we pass an invalid pointer to _test_free() function, this function will be crashed. So I update this function, add some checking code. Therefore, cmockery can test cases that free invaild pointer, and free an allocated block twice.

[cryptomilk]

cmockery is dead since several years, you probably want to take a look at https://cmocka.org which is the successor of cmockery. We are happy to accept patches to improve it.