Do not save assets in tree list calls

[markus-moser]

If image dimensions cannot be calculated the core will save the asset again and again in all tree list calls:

https://github.com/pimcore/pimcore/blob/master/bundles/AdminBundle/Controller/Admin/Asset/AssetController.php#L717

// this is for backward-compatibility, to calculate the dimensions if they are not there
if (!$asset->getCustomSetting('imageDimensionsCalculated')) {
        $asset->save();
}

This might happen for example if the asset is somehow corrupt or not in the file system.