small improvement for fill and fill_n

[Paolo Carlini]

Nathan Myers wrote:

>As always, the "as-if" rule applies: if user program can't detect
>the difference, then it's OK.  In some cases (when it says in the
>standard, explicitly, "as-if") it's OK even though it's possible to
>write user code that can tell, as long as the postconditions are
>satisfied.
>  
>
Indeed ;)

Sorry for not ackowledging earlier your comment...

Paolo.