Fix pve script when no net or disk stats are available

zabbix_scripts/check_pve_sudo

@@ -67,10 +67,10 @@ if ($cluster){
 
   my $guests = from_json(qx($pvesh get /cluster/resources --type=vm $pvesh_opt 2>/dev/null));
   foreach my $guest (@{$guests}){
-    $json->{network}->{in}  += $guest->{netin};
-    $json->{network}->{out} += $guest->{netout};
-    $json->{disk}->{read}   += $guest->{diskread};
-    $json->{disk}->{write}  += $guest->{diskwrite};
+    $json->{network}->{in}  += $guest->{netin}     || 0;
+    $json->{network}->{out} += $guest->{netout}    || 0;
+    $json->{disk}->{read}   += $guest->{diskread}  || 0;
+    $json->{disk}->{write}  += $guest->{diskwrite} || 0;
   }
 } elsif ($node){
   foreach my $item (qw(status version subscription)){