fastbinary.c python deserialize can cause huge allocations from garbage

In the fastbinary python deserializer, allocating a list is done like so:

    len = readI32(input);
    if (!check_ssize_t_32(len)) {
      return NULL;
    }

    ret = PyList_New(len);

The only validation of len is that it's under INT_MAX. I've encountered a situation where upon receiving garbage input, and having len be read as something like 1 billion, the library treated this as a valid input, allocated gigs of RAM, and caused a server to crash.

The quick fix I made was to limit list sizes to a sane value of a few thousands that more than suits my personal needs.

But IMO this should be dealt with properly. One way that comes to mind is not pre-allocating the entire list in advance in case it's really big, and resizing it in smaller steps while reading the input.