viewfeed: check if feed exists and report error if not

backend.php

@@ -721,6 +721,15 @@
 			}
 		}
 
+		$result = db_query($link,
+			"SELECT id FROM ttrss_feeds WHERE id = '$feed' LIMIT 1");
+		
+		if (db_num_rows($result) == 0) {
+			print "<div align='center'>
+				Could not display feed: feed not found.</div>";
+			return;
+		}
+
 		if (preg_match("/^-?[0-9][0-9]*$/", $feed) != false) {
 	
 			$result = db_query($link, "SELECT rtl_content FROM ttrss_feeds