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.
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