Detect UTF-16 surrogates in `isc_utf8_valid()`

Detect UTF-16 surrogates in `isc_utf8_valid()`

UTF-8 standard forbid usage of unicode character between the range of
0xD800..0xDFFF (reserved, and used as UTF-16 surrogates, see RFC 3629).

However, `usc_utf8_valid()` was not checking if the encoded unicode
character was in this range, which then would accept invalid UTF-8
strings. This is now fixed.