Fix zero size debug array swap noexcept qualification

[François Dumont]

As it was trivial and only impacting debug mode that I maintain 
officially I have commited on the day I sent the email.

I hope it is ok.

François


On 27/06/2016 14:46, Jonathan Wakely wrote:
> On 23/06/16 22:22 +0200, François Dumont wrote:
>>    Debug mode array had simply been forgotten when fixing zero-size 
>> swap method as part of swappable traits implementation.
>>
>>    * include/debug/array (array<>::swap): Fix noexcept qualificaton for
>>    zero-size array.
>>
>>    Tested under Linux x86_64 debug mode.
>
> OK for trunk, thanks.
>
>