Fix bug on avatar (#31008) (#31019)

Backport #31008 by @lunny

Extract from #30995

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/api/v1/user/avatar.go

@@ -39,6 +39,7 @@ func UpdateAvatar(ctx *context.APIContext) {
 	err = user_service.UploadAvatar(ctx, ctx.Doer, content)
 	if err != nil {
 		ctx.Error(http.StatusInternalServerError, "UploadAvatar", err)
+		return
 	}
 
 	ctx.Status(http.StatusNoContent)
@@ -57,6 +58,7 @@ func DeleteAvatar(ctx *context.APIContext) {
 	err := user_service.DeleteAvatar(ctx, ctx.Doer)
 	if err != nil {
 		ctx.Error(http.StatusInternalServerError, "DeleteAvatar", err)
+		return
 	}
 
 	ctx.Status(http.StatusNoContent)