Keep local cached images fresh if file exists.

Updates the cache_images() function to `touch` images if the file exists already. Some sites (e.g. news) re-use stock images and this change updates the local file's modified time every time the image is referenced in new articles. This should prevent the local file from being expired/deleted only to be re-cached later on.

include/rssfuncs.php

@@ -1248,6 +1248,8 @@
 					if ($file_content && strlen($file_content) > _MIN_CACHE_IMAGE_SIZE) {
 						file_put_contents($local_filename, $file_content);
 					}
+				} else {
+					touch($local_filename);
 				}
 			}
 		}