sms: initialize 'validity' property always to a default

When we're exposing not-yet-completed multipart messages, we need to provide a
correct value for the validity property, or gdbus may crash, see e.g.:

https://bugzilla.gnome.org/show_bug.cgi?id=704319

src/mm-sms.c

@@ -1665,6 +1665,9 @@ mm_sms_multipart_new (MMBaseModem *modem,
                   MM_SMS_MULTIPART_REFERENCE, reference,
                   "state",                    state,
                   "storage",                  storage,
+                  "validity",                 g_variant_new ("(uv)",
+                                                             MM_SMS_VALIDITY_TYPE_UNKNOWN,
+                                                             g_variant_new_boolean (FALSE)),
                   NULL);
 
     if (!mm_sms_multipart_take_part (self, first_part, error))