Fix NULL-ptr derefs for container without rootfs

Fix NULL-ptr derefs for container without rootfs

Since we allow containers to be created without a rootfs most checks in conf.c
are not sane anymore. Instead of just checking if rootfs->path != NULL we need
to check whether rootfs != NULL.

Minor fixes:
- Have mount_autodev() always return -1 on failure: mount_autodev() returns 0
  on success and -1 on failure. But when the return value of safe_mount() was
  checked in mount_autodev() we returned false (instead of -1) which caused
  mount_autodev() to return 0 (success) instead of the correct -1 (failure).

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>