properly check for entry id array being present before iterating it in mobile/classic (closes #267)

mobile/classic/functions.php

@@ -364,6 +364,7 @@
 		}
 
 		if ($catchup_op == "selection") {
+			if (is_array($_GET["sel_ids"])) {
 				$ids_to_mark = array_keys($_GET["sel_ids"]);
 				if ($ids_to_mark) {
 					foreach ($ids_to_mark as $id) {
@@ -373,6 +374,7 @@
 					}
 				}
 			}
+		}
 
 		if ($subop == "MarkPageRead" || $catchup_op == "page") {
 			$ids_to_mark = $_SESSION["last_page_ids.$feed"];