Prevent strange crashes in fmt with absurd goal lengths introduced by the

Prevent strange crashes in fmt with absurd goal lengths introduced by the
support for wide characters.

If the sizeof (wchar_t) times max_length would yield a value beyond
representation in a size_t, exit with a usage error up front, rather than
strange errors down the line from trying to malloc (well, realloc) with a size
of 0.

This is perhaps not the optimal behaviour - a clamp may be more appropriate as
we clamp the value of max_length now anyway, but this is at least better than
segfaulting or worse.  On systems which are friendly to malloc with a value of 0
the results could end up being strange corruption of the output.