fix(Calendar VCalendar) improve attachment filename handling

Change-Id: I605ef3683e619399a0697d4fa76402c0b9438a3b
Reviewed-on: http://gerrit.tine20.com/customers/18673
Tested-by: Jenkins CI (http://ci.tine20.com/) <tine20-jenkins@metaways.de>
Reviewed-by: Paul Mehrer <p.mehrer@metaways.de>

tine20/Calendar/Convert/Event/VCalendar/Abstract.php

@@ -460,8 +460,10 @@ protected function _addAttachments($vevent, $vcalendar, $event)
      */
     protected function _getAttachmentFilename($string)
     {
-        $filename = str_replace([' ', '/'], '_', $string);
-        $filename = iconv("UTF-8", "ascii//TRANSLIT", $filename);
+        $string = str_replace([' ', '/'], '_', $string);
+        if (false === ($filename = @iconv("UTF-8", "ascii//TRANSLIT", $string))) {
+            $filename = iconv("UTF-8", "ascii//IGNORE", $string);
+        }
 
         return $filename;
     }