Fletcher32 raises IndexError on empty buffers #888

Fletcher32 raises IndexError when it encodes an empty buffer, and when it decodes a 4-byte buffer that holds only the checksum. The other checksum codecs (CRC32, Adler32, JenkinsLookup3) round-trip empty input without error.

Reproduce

from numcodecs import Fletcher32

Fletcher32().encode(b"")
# IndexError: Out of bounds on buffer access (axis 0)

Fletcher32().decode(b"\x00\x00\x00\x00")
# IndexError: Out of bounds on buffer access (axis 0)

Tested on main (54fc274), Python 3.13, NumPy 2.

Cause

fletcher32.pyx takes &mv[0] of the payload memoryview in _fletcher32, encode, and the out branch of decode. Cython bounds-checks that index, so it fails when the memoryview is empty.

Expected

An empty payload has a Fletcher-32 checksum of 0, the same result the HDF5/netCDF reference implementation gives for a zero-length input. encode(b"") returns b"\x00\x00\x00\x00", and decoding that buffer returns an empty buffer.