backend/pubsub: better 404 handling, do not cause PHP warning when invalid feed id is being requested

backend.php

@@ -553,6 +553,8 @@
 			$result = db_query($link, "SELECT feed_url FROM ttrss_feeds
 				WHERE id = '$feed_id'");
 
+			if (db_num_rows($result) != 0) {
+
 				$check_feed_url = db_fetch_result($result, 0, "feed_url");
 
 				if ($check_feed_url && ($check_feed_url == $feed_url || !$feed_url)) {
@@ -581,6 +583,11 @@
 					}
 				} else {
 					header('HTTP/1.0 404 Not Found');
+					echo "404 Not found";
+				}
+			} else {
+				header('HTTP/1.0 404 Not Found');
+				echo "404 Not found";
 			}
 
 		break; // pubsub