gh-148688: Fix _BlocksOutputBuffer_Finish() double free (#148689)

If _BlocksOutputBuffer_Finish() fails (memory allocation failure),
PyBytesWriter_Discard() is called on the writer. Then if
_BlocksOutputBuffer_OnError() is called, it calls again
PyBytesWriter_Discard() causing a double free.

Fix _BlocksOutputBuffer_Finish() by setting buffer->writer to NULL,
so _BlocksOutputBuffer_OnError() does nothing instead of calling
PyBytesWriter_Discard() again.