uvoffuni_to_utf8_flags() die if platform can't handle

uvoffuni_to_utf8_flags() die if platform can't handle

On non EBCDIC platforms currently any UV is encodable as UTF-8.  (This
would change if there were 128-bit words).  Thus, much code assumes that
nothing can go wrong when converting to UTF-8, and hence does no error
checking.

However, UTF-EBCDIC is only capable of representing code points below
2**32, so if there are 64-bit words, this function can fail.

Prior to this patch, there was no real overflow check, and garbage was
returned by this function if called with too large a number.

While not ideal, the easiest thing to do is to just die for such a
number, like we do for division by 0.  This involves changing only code
within this function, and not its many callers.