message: Don't assume there's a sub-message

Also add unit-test.

Fixes: issue #2256.

lib/message/mu-message-part.cc

@@ -67,9 +67,12 @@ MessagePart::cooked_filename() const noexcept
 	// MimeMessagepart. Construct a name based on subject.
 	if (mime_object().is_message_part()) {
 		auto msg{MimeMessagePart{mime_object()}.get_message()};
-		return msg.subject()
+		if (!msg)
-			.map(cleanup)
+			return Nothing;
-			.value_or("no-subject") + ".eml";
+		else
+			return msg->subject()
+				.map(cleanup)
+				.value_or("no-subject") + ".eml";
 	}
 
 	return Nothing;

lib/message/mu-message.cc

@@ -417,7 +417,10 @@ process_message_part(const MimeMessagePart& msg_part,
 		     Message::Private& info)
 {
 	auto submsg{msg_part.get_message()};
-	submsg.for_each([&](auto&& parent, auto&& child_obj) {
+	if (!submsg)
+		return;
+
+	submsg->for_each([&](auto&& parent, auto&& child_obj) {
 
 		/* XXX: we only handle one level */
 

lib/message/mu-mime-object.hh

@@ -1182,11 +1182,14 @@ public:
 	/**
 	 * Get the MimeMessage for this MimeMessagePart.
 	 *
-	 * @return the MimeMessage
+	 * @return the MimeMessage or Nothing
 	 */
-	MimeMessage get_message() const {
+	Option<MimeMessage> get_message() const {
-		return MimeMessage(
+		auto msg{g_mime_message_part_get_message(self())};
-			Object(G_OBJECT(g_mime_message_part_get_message(self()))));
+		if (msg)
+			return MimeMessage(Object(G_OBJECT(msg)));
+		else
+			return Nothing;
 	}
 private:
 	GMimeMessagePart* self() const {

lib/message/test-mu-message.cc

@@ -456,6 +456,22 @@ C0bdoCx44QVU8HaZ2x91h3GoM/0q5bqM/rvCauwbokiJgAUrznecNPY=
 	g_assert_cmpuint(n, ==, 1);
 }
 
+
+static void
+test_message_multipart_mixed_rfc822(void)
+{
+	constexpr const char *msgtext =
+R"(Content-Type: multipart/mixed;
+	boundary="Multipart_Tue_Sep__2_15:42:35_2014-1"
+
+--Multipart_Tue_Sep__2_15:42:35_2014-1
+Content-Type: message/rfc822
+)";
+	auto message{Message::make_from_text(msgtext)};
+	g_assert_true(!!message);
+}
+
+
 int
 main(int argc, char* argv[])
 {
@@ -469,6 +485,8 @@ main(int argc, char* argv[])
 			test_message_signed);
 	g_test_add_func("/message/message/signed-encrypted",
 			test_message_signed_encrypted);
+	g_test_add_func("/message/message/multipart-mixed-rfc822",
+			test_message_multipart_mixed_rfc822);
 
 	return g_test_run();
 }