make OUString::copy() clip

[Noel Grandin]

On 2012-08-02 14:35, Stephan Bergmann wrote:
>
> No "defensive programming," please.  If we consider a given 
> (beginIndex, count) arguments a violation of copy's precondition, then 
> fail on it, as fast as possible (via assert -> abort in a non-NDEBUG 
> build, via undefined behavior (-> hopefully crash soon) otherwise).
Rather crash earlier than later.
   *NULL = 0xdeadbeef;
can't be all that expensive.

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