libutil/iconv: avoid overflow in surrogate pairs

libutil/iconv: avoid overflow in surrogate pairs

Consider the non-conforment utf-8 sequence "\xf5\x80\x80\x80", which
would encode 0x140000. We would set the high byte of the first
surrogate to 0xd8 | (0x130000 >> 18), or 0xdc, which is an invalid
start for a high surrogate, making the sequence as a whole invalid (as
you would expect -- the Unicode range was set precisely to that
covered by utf-16 surrogates).

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>