cocos2d::Data: prevent use after free.

[PatriceJiang]

prevent free _bytes when src and dest are identical.

[minggo]

@PatriceJiang what problem does this PR fix? Could you please add a test case for it?

[PatriceJiang]

it is not really a bug.
just protect rare cases like this

Data d;
d.copy(d.getBytes(), len); //expect to be no side effect

[minggo]

I think it is more reasonable to protect in clear(), like this

void Data::clear()
{
    if (_bytes)
        free(_bytes);

    _bytes = nullptr;
    _size = 0;
}

[minggo]

Why need to set to nullptr here? It seems it has not effect.

[PatriceJiang]

when size == 0 and bytes == _bytes, _bytes should be pointed to nullptr

[minggo]

But if size == 0, then it means doesn't copy anything.

[PatriceJiang]

yes, this is a better way

[Dimon4eg]

Now in copy memory leak possible:

Data d;
int buff[3] = {1, 2, 3};
d.copy(buff, 3); // memory allocated
d.copy(d.getBytes(), 0); //leak here

[PatriceJiang]

1. prevent copy when size is negative
2. no copy when bytes == _bytes

ssize_t Data::copy(const unsigned char* bytes, const ssize_t size)
{
    CCASSERT(size >= 0, "copy size should be non-negative");

    if (size <= 0) return 0;

    if (bytes != _bytes)
    {
        clear();
    }
    else
    {
        _size = size;
        return _size;
    }
    
    if (size > 0)
    {
        _bytes = (unsigned char*)malloc(sizeof(unsigned char) * _size);
        memcpy(_bytes, bytes, _size);
    }
    _size = size;
    return _size;
}

@Dimon4eg what do you think?

[Dimon4eg]

Much better. Just don't like return in the middle of the function.

[Dimon4eg]

Perfect!