Documentation for _mongoc_buffer_try_append_from_stream() states:

 * @error: A location for a bson_error_t, or NULL.
...
 * Returns: bytes read if successful; otherwise -1 and @error is set.

However, the error variable is never used within the function. AFAICT, this function is only called from _mongoc_async_cmd_phase_recv_len() and _mongoc_async_cmd_phase_recv_rpc(), both of which set the error struct on their own if _mongoc_buffer_try_append_from_stream() returns 0 or -1.

It looks like this argument can simply be removed.