Fix out-of-bounds read on non-contiguous memoryview in pack_command

[Usamak06]

pack_command copies p_buf->len bytes from the base pointer PyMemoryView_GET_BUFFER returns, which only spans the payload when the view is C-contiguous; a reversed or strided memoryview puts buf at a different offset and carries its own strides, so the copy runs off the end of the underlying buffer and the out-of-bounds bytes end up in the packed command. Packing memoryview(bytearray(b'.' * 32))[::-1] returns 15 bytes belonging to the next heap allocation, and a 256 MiB bytearray segfaults. Acquiring the view with PyObject_GetBuffer under PyBUF_SIMPLE rejects non-contiguous input with the same BufferError that Reader.feed already raises for it and leaves contiguous memoryviews unchanged; the added test fails before the change, and the suite passes on CPython 3.11 and PyPy 3.11 with and without NDEBUG.

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Note

High Risk
Fixes a memory-safety bug (OOB read / possible segfault and leaking adjacent heap bytes into packed commands) in command serialization.

Overview
Fixes an out-of-bounds read when pack_command packs a non-contiguous memoryview (e.g. reversed or strided). The old path used PyMemoryView_GET_BUFFER and copied len bytes from buf as if the view were C-contiguous, which could read past the underlying buffer and put garbage (or crash) into the serialized Redis command.

Memoryview arguments are now acquired via PyObject_GetBuffer(..., PyBUF_SIMPLE) with error handling and PyBuffer_Release, so non-contiguous views raise BufferError instead of being packed incorrectly. Contiguous memoryviews behave as before.

A regression test packs a reversed memoryview and expects BufferError.

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