ttyplot.c: Fix crash with -O1 on Ctrl+C

To reproduce:

1. Run "ttyplot" (as is)
2. Hit Ctrl+C
3. See message "*** bit out of range 0 - FD_SETSIZE on fd_set ***: terminate"
   and "Aborted"

ttyplot.c

@@ -481,7 +481,7 @@ static int wait_for_events(int signal_read_fd, int tty, bool stdin_is_open, stru
             ret |= EVENT_SIGNAL_READABLE;
         }
 
-        if (FD_ISSET(tty, &read_fds)) {
+        if ((tty != -1) && FD_ISSET(tty, &read_fds)) {
             ret |= EVENT_TTY_READABLE;
         }