Verify existence of $_GET key

Otherwise when the file without any specified mimetype was accessed the error log was flooded with entries such as "Undefined index: mime", there can be multiple issues found about this in the forum and our bugtracker.

To test this access `/index.php/apps/files/ajax/mimeicon.php` with and without `$_GET['mime']`.

Fixes itself.

apps/files/ajax/mimeicon.php

@@ -1,4 +1,6 @@
 <?php
 \OC::$server->getSession()->close();
 
-print OC_Helper::mimetypeIcon($_GET['mime']);
+$mime = isset($_GET['mime']) ? $_GET['mime'] : '';
+
+print OC_Helper::mimetypeIcon($mime);