Invalid keycodes also return 0 (but print a msg in DEBUG).

src/event_manager.h

@@ -53,7 +53,6 @@ void handle_event();
  *
  * @param code The key code.
  *
- * @return If down 1 will be returned, if the keycode is
- * invalid then -1 is returned, else 0.
+ * @return If down 1 will be returned, else 0.
  */
 int key_is_down(int code);