gh-156939: Fix struct.pack('0p', bytes)

[vstinner]

If the Pascal string is empty (size=0), do not write the size prefix. Previously, a NUL byte was written outsize the buffer (buffer overflow).

• Issue: C API: Detect buffer overflow in PyBytesWriter #156939

[vstinner]

In Python 3.14 and older, struct.pack('0p', b'abc') writes a NUL byte after the bytes contents. But bytes objects allocates an extra byte for a trailing NUL byte, so the buffer overflow doesn't write arbitrary memory. It's just a silent write which doesn't corrupt anything.

On Python 3.15, struct.pack() uses PyBytesWriter which doesn't allocate an extra trailing NUL byte for small strings (up to 256 bytes). And so a buffer overflow can write outsize PyBytesWriter.small_buffer. In practice, it should write a NUL byte in ``PyBytesWriter.objwhich is aNULL` pointer, so it should not corrupt arbitrary memory.

Well, it's better to avoid a buffer overflow anyway :-)