`chunksize == 0`

**Describe the bug**

I'm puzzled by `chunksize == 0` in this piece of code:
https://github.com/Blosc/c-blosc2/blob/b2434ad1be6ea46e101e0691bfd6b2a43174cc8a/blosc/frame.c#L2424-L2426

* if `chunksize == 0`, then `chunk_nbytes < (size_t)chunksize)` is equivalent to `chunk_nbytes < 0)`
* `chunksize == 0` does not seem consistent with the `Check that we are not appending a small chunk after another small chunk` comment, but I may be wrong here.

Start by reading frame.c at lines 2424-2426 and trace how chunksize and chunk_nbytes are set before this check. Determine whether chunksize == 0 is valid there and whether the condition matches its comment; done means the behavior and explanation are consistent.

Written by the indexing model from the issue text.