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 b823809bca and
cf89e48028.

src/shared/ptyfwd.c

@@ -810,7 +810,7 @@ static int shovel_force(PTYForward *f) {
 
         if (!f->master_hangup)
                 f->master_writable = f->master_readable = true;
-        if (!f->stdin_hangup)
+        if (!f->stdin_hangup && f->input_fd >= 0)
                 f->stdin_readable = true;
         if (!f->stdout_hangup)
                 f->stdout_writable = true;