system: add latency_avg as sanity check for running dpinger; closes #10937

The check was introduced in 0df1c0d so there's no technical background
to take into account.  Indeed, latency_avg is probably the reading that
would least likely be 0 in this case.

Suggested by: @mbccd

src/etc/inc/plugins.inc.d/dpinger.inc

@@ -350,7 +350,7 @@ function dpinger_status()
         fclose($fp);
 
         list(, $latency_avg, $latency_stddev, $loss) = explode(' ', preg_replace('/\n/', '', $dinfo));
-        if ($latency_stddev == '0' && $loss == '0') {
+        if ($latency_avg == '0' && $latency_stddev == '0' && $loss == '0') {
             continue;
         }