ptyfwd: do not try to read from PTYForward.input_fd when read-only mode

ptyfwd: do not try to read from PTYForward.input_fd when read-only mode

Fixes the following error message (the last line):
```
[FAILED] Failed to start TEST-60-MOUNT-RATELIMIT.service.
Sending SIGTERM to remaining processes...
Sending SIGKILL to remaining processes...
All filesystems, swaps, loop devices, MD devices and DM devices detached.
Exiting container.
Failed to read from pty input fd: Bad file descriptor
```

Follow-up for b823809bca6cd531a54fcf0f02427aea7cd3e651 and
cf89e48028ed3b651f63ba624175394c3c0d8efa.