Fix utf8.RuneError test

arp242/uni@5f9eb0d

This is actually not really an issue here, because U+FFFD isn't valid in
TOML, but it doesn't hurt to properly check.

lex.go

@@ -166,7 +166,7 @@ func (lx *lexer) next() (r rune) {
 	}
 
 	r, w := utf8.DecodeRuneInString(lx.input[lx.pos:])
-	if r == utf8.RuneError {
+	if r == utf8.RuneError && w == 1 {
 		lx.error(errLexUTF8{lx.input[lx.pos]})
 		return utf8.RuneError
 	}