Segfault in `ZstdCompressorIterator_iternext` with buffer-protocol data

Nobody has claimed this yet.

Hi,

I've been experimenting with passing `memoryview`s into `read_to_iter`, and I've been hitting segfaults pretty regularly on large pieces of data.

The offending line is `Py_DECREF(self->readResult)` in `compressoriterator.c`. For input data passed in as buffer objects, this pointer (`self->readResult`) is never not-null.

I've been unable to create a small re-producer which hits these conditions:

```c
if (self->input.pos < self->input.size) {
...

if (self->input.pos == self->input.size) {
...
Py_DECREF(self->readResult);
...
}
```

But I'll keep trying.

Start in compressoriterator.c at ZstdCompressorIterator_iternext and inspect the Py_DECREF(self->readResult) path when read_to_iter receives buffer-protocol data such as memoryview. Work toward a small reproducer for large inputs, then verify that the reproducer no longer segfaults and that the completed iteration remains safe.

Written by the indexing model from the issue text.