Improve user nick validation

arn/validate/Validate.go

@@ -5,6 +5,7 @@ import (
 	"regexp"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/animenotifier/notify.moe/arn/autocorrect"
 )
@@ -31,6 +32,10 @@ func Nick(nick string) bool {
 		return false
 	}
 
+	if !utf8.ValidString(nick) {
+		return false
+	}
+
 	return nick == autocorrect.UserNick(nick)
 }