fix utf8.DecodeRune error condition check

On error it's documented as returning "RuneError, 1" (which is
incorrect/wrong, since RuneError has size 2). But we didn't check that
second return value, so when you actually *wanted* RuneError it didn't
work.

Fixes #46

uni.go

@@ -773,7 +773,7 @@ func print(args []string, format string, raw bool, as printAs) error {
 			}
 
 			r, s := utf8.DecodeRune(byt)
-			if r == utf8.RuneError {
+			if r == utf8.RuneError && s == 1 {
 				return fmt.Errorf("invalid UTF-8 sequence: %q", a)
 			}
 			if s != len(byt) {