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

[Roland McGrath]

> 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.