api/repo: check user type of owner for migration

Report by MiaoWoo.

routers/api/v1/repo/repo.go

@@ -220,6 +220,9 @@ func Migrate(ctx *context.APIContext, f form.MigrateRepo) {
 				ctx.Error(500, "GetUserByID", err)
 			}
 			return
+		} else if !org.IsOrganization() {
+			ctx.Error(403, "", err)
+			return
 		}
 		ctxUser = org
 	}