citrus: Avoid invalid code points.

From the OpenBSD log:
The UTF-8 decoder should not accept byte sequences which decode to unicode
code positions U+D800 to U+DFFF (UTF-16 surrogates), U+FFFE, and U+FFFF.

http://www.cl.cam.ac.uk/~mgk25/unicode.html#utf-8
http://unicode.org/faq/utf_bom.html#utf8-4

Reported by:	Stefan Sperling
Obtained from:	OpenBSD
MFC after:	5 days