Update bufferevent_filter.c

In function 
static int
be_filter_ctrl(struct bufferevent *bev, enum bufferevent_ctrl_op op,
    union bufferevent_ctrl_data *data)
{
	struct bufferevent_filtered *bevf;
	switch (op) {
	case BEV_CTRL_GET_UNDERLYING:
		bevf = upcast(bev);
		data->ptr = bevf->underlying;
		return 0;
	case BEV_CTRL_GET_FD:
	case BEV_CTRL_SET_FD:
	case BEV_CTRL_CANCEL_ALL:
	default:
		return -1;
	}
}

upcast api may return null and we are trying to dereference bevf (bufferevent_filtered)
To resolve the issue null check added for bevf