Issues with user-defined codecs

I'm facing issues when creating a simple codec that just makes a copy of the data to get familiar with Blosc's registering machinery. I attach the code:

```python
import blosc2
import numpy as np

# Create an User-defined codec (just a memcpy)

def encoder(input, output, meta, schunk: blosc2.SChunk):
print(f"Encoder output size: {output.size}")
output[:schunk.blocksize] = input[:schunk.blocksize]
return schunk.blocksize

def decoder(input, output, meta, schunk: blosc2.SChunk):
output[:schunk.blocksize] = input[:schunk.blocksize]
return schunk.blocksize

# Register the codec
codec_id = 200
blosc2.register_codec('test1', codec_id, encoder, decoder)

# Compress this array with the new codec

shape = (100, 100)
a = np.ones(shape, dtype=np.int64)

cparams = {
'codec': codec_id,
'nthreads': 1,
'filters': [],
'splitmode': blosc2.SplitMode.NEVER_SPLIT,
}

dparams = {
'nthreads': 1,
}

chunks = shape
blocks = (50, 50)

c_a = blosc2.asarray(a, chunks=chunks, blocks=blocks, cparams=cparams, dparams=dparams)
```

However, when I run the previous code, I get the following:
```
Encoder output size: 20000
Encoder output size: 20000
Encoder output size: 20000
Encoder output size: 19968

ValueError: could not broadcast input array from shape (20000,) into shape (19968,)
```

Looking at this, it appears that the last block of the chunk is smaller than the others. Do you know what is happening? Is there something I'm doing wrong?