server : do not return error when running out of context (with ctx shift disabled)

[ngxson]

Currently, when context shifting is disabled, we trigger the line of code that is not supposed to be called:

if (!params_base.ctx_shift) {
    // this check is redundant (for good)
    // we should never get here, because generation should already stopped in process_token()
    slot.release();
    send_error(slot, "context shift is disabled", ERROR_TYPE_SERVER);
    continue;
}

This returns an error, which makes downstream applications unhappy.

NOTE: some variables may need to be renamed after #13576