`ZstdCompressionWriter()` constructs successfully but leads to `segmentation fault`

I've been fuzzing Python C extension modules for a small research project.
I found a sanitizer issue which is a null-pointer dereference in ZstdCompressionWriter_memory_size.

I reproduced it with the binary wheel from a plain pip install zstandard.
The process terminates with SIGSEGV there as well.

I'm not sure whether zero-argument construction of this type is considered supported (calling ZstdCompressionWriter() with no arguments),
but since it currently terminates the interpreter rather than raising a Python exception, I thought it was worth reporting.

Versions

zstandard 0.25.0, cext backend, CPython 3.12, Linux x86_64.

Reproducer

import zstandard

w = zstandard.ZstdCompressionWriter()   # succeeds
w.memory_size()                         # SIGSEGV

Five calls across the two writer types behave the same way, each from a fresh direct construction:

Sanitizer build

Same call, zstandard built with -fsanitize=address,undefined:

c-ext/compressionwriter.c:64:65: runtime error:
    member access within null pointer of type 'ZstdCompressor'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
    c-ext/compressionwriter.c:64:65

AddressSanitizer: SEGV on unknown address 0x000000000020
    #0 ZstdCompressionWriter_memory_size  c-ext/compressionwriter.c:64:65

ZstdDecompressionWriter.memory_size() reports the equivalent at
c-ext/decompressionwriter.c:50:67.

Expected behavior

I think that direct construction should be rejected with a Python exception, as it is by the CFFI backend.

Under PYTHON_ZSTANDARD_IMPORT_POLICY=cffi the same construction is refused outright:

TypeError: ZstdCompressionWriter.__init__() missing 5 required positional
arguments: 'compressor', 'writer', 'source_size', 'write_size',
and 'write_return_read'

Actual behavior

The C extension permits construction with no arguments, leaving internal fields NULL.
Several methods dereference those fields and terminate the process with SIGSEGV.

Although these objects are normally obtained through stream_writer(), I wonder whether the C extension should reject zero-argument construction, as the CFFI backend already does, rather than produce an uninitialized object.

Führe zuerst den Python-Reproducer aus und untersuche dann c-ext/compressionwriter.c:64 und c-ext/decompressionwriter.c:50 zusammen mit den Pfaden zur Writer-Konstruktion. Vergleiche das Verhalten der C-Erweiterung bei der Konstruktion ohne Argumente mit dem CFFI-Backend. Die Aufgabe ist abgeschlossen, wenn die direkte Konstruktion keine erreichbaren internen NULL-Felder mehr hinterlässt und statt eines Segmentation Faults eine Python-Ausnahme auslöst.