Add a regression test for Bug#30004.

* test/src/callint-tests.el
(call-interactively/incomplete-multibyte-sequence): New unit test.

test/src/callint-tests.el

+;;; callint-tests.el --- unit tests for callint.c    -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2018 Free Software Foundation, Inc.
+
+;; Author: Philipp Stephani <phst@google.com>
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Unit tests for src/callint.c.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest call-interactively/incomplete-multibyte-sequence ()
+  "Check that Bug#30004 is fixed."
+  (let ((data (should-error (call-interactively (lambda () (interactive "\xFF"))))))
+    (should
+     (equal
+      (cdr data)
+      '("Invalid control letter `\u00FF' (#o377, #x00ff) in interactive calling string")))))
+
+;;; callint-tests.el ends here