Avoid leaking fds on evconnlistener with no callback set

There's no way to retrieve an fd from an evconnlistener whose
callback has been cleared, so we had better close any such fd.

Found by coverity; CID 739725.

listener.c

@@ -407,6 +407,7 @@ listener_read_cb(evutil_socket_t fd, short what, void *p)
 			evutil_make_socket_nonblocking(new_fd);
 
 		if (lev->cb == NULL) {
+			evutil_closesocket(new_fd);
 			UNLOCK(lev);
 			return;
 		}