fix 500 error for ghost avatar

routers/user/avatar.go

@@ -6,6 +6,7 @@ package user
 
 import (
 	"strconv"
+	"strings"
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/context"
@@ -23,7 +24,9 @@ func Avatar(ctx *context.Context) {
 
 	log.Debug("Asked avatar for user %v and size %v", userName, size)
 
-	user, err := models.GetUserByName(userName)
+	user := new(models.User)
+	if strings.ToLower(userName) != "ghost" {
+		user, err = models.GetUserByName(userName)
 		if err != nil {
 			if models.IsErrUserNotExist(err) {
 				ctx.ServerError("Requested avatar for invalid user", err)
@@ -32,6 +35,9 @@ func Avatar(ctx *context.Context) {
 			}
 			return
 		}
+	} else {
+		user = models.NewGhostUser()
+	}
 
 	ctx.Redirect(user.RealSizedAvatarLink(size))
 }