deque fails to meet 23.2.1.3.4-5 causing invalid iterators and extra assignments

[Paolo Carlini]

Paolo Carlini wrote:

> Coming then to your proposed solution, I don't like that it 
> effectively removes the nice idea of always copying the  optimal 
> (minimum) number of elements: AFAICS, only comparing to (size() - __n) 
> / 2 exactly achieves that.

Never mind, sorry, now I see that the two calculations should be 
equivalent, only matter of taste...

I look forward for your other comments, then we can commit either solution.

Thanks again,
Paolo.