gRPC: clearing buffers in ngx_http_grpc_get_buf(). We copy input buffers to our buffers, so various flags might be unexpectedly set in buffers returned by ngx_chain_get_free_buf(). In particular, the b->in_file flag might be set when the body was written to a file in a different context. With sendfile enabled this in turn might result in protocol corruption if such a buffer was reused for a control frame. Make sure to clear buffers and set only fields we really need to be set.