(systemhealth) don't generate crash report on empty or invalid rrd xml data

src/opnsense/mvc/app/controllers/OPNsense/Diagnostics/Api/SystemhealthController.php

@@ -542,13 +542,11 @@ class SystemhealthController extends ApiControllerBase
          */
 
         $rrd_details=$this->getRRDdetails($rrd)["data"];
-
+        $xml = false;
         if ($rrd_details['filename'] != "") {
             $backend = new Backend();
             $response = $backend->configdpRun("systemhealth fetch ", array($rrd_details['filename']));
-            $xml = simplexml_load_string($response);	/* XXX can cause a crash report */
-        } else {
-            $xml = false;
+            $xml = @simplexml_load_string($response);
         }
 
         if ($xml !== false) {