core/automount: do not abort when there is no autofs to talk to

Releasing the requests the kernel queued goes through `open_ioctl_fd()`, which
asserts on the manager fd for /dev/autofs. That fd stays unset when
`open_dev_autofs()` fails while coldplugging an automount, while the
deserialized pipe and its tokens outlive it.

Report the failure to the caller instead of asserting/crashing PID 1.

Signed-off-by: Martin Pitt <martin@amutable.com>

src/core/automount.c

@@ -458,6 +458,11 @@ static int automount_send_ready(Automount *a, Set *tokens, int status) {
         if (set_isempty(tokens))
                 return 0;
 
+        /* We have nothing to talk to the autofs with, e.g. because open_dev_autofs() failed at coldplug and
+         * left us with the deserialized pipe and tokens. Report that instead of asserting below. */
+        if (UNIT(a)->manager->dev_autofs_fd < 0 || !a->where)
+                return -EBADF;
+
         ioctl_fd = open_ioctl_fd(UNIT(a)->manager->dev_autofs_fd, a->where, a->dev_id);
         if (ioctl_fd < 0)
                 return ioctl_fd;