Fix localedef collation handling of <U0000> (bug 15948)

[Joseph S. Myers]

On Tue, 24 Sep 2013, Roland McGrath wrote:

> > as can be seen by adding "assert (runp->nwcs > 0);" before that code,
> 
> So add that assert.
> 
> > This patch causes such a sequence to be treated as length 1 instead.
> 
> This seems reasonable on its face.  But I don't understand enough about the
> context to be sure how this case comes about or what it ought to mean.
> It would be much easier to evaluate with an example of how the case occurs.

The examples are the cases where the assertion fails - such as 
ar_SA.UTF-8.  I don't know *why* locales/ar_SA includes <U0000> in 
LC_COLLATE (more generally, I'm not sure how much individual locales need 
their own collation information at all, as opposed to using the ISO 14651 
information, which, as noted in bug 14095, could do with someone figuring 
out how to update it), but it's the appearance of collation information 
for <U0000> in the locale sources that triggers this problem.

-- 
Joseph S. Myers
joseph@codesourcery.com