access: dvdnav: check directory content on probing

If we probe directory, check if it contains VIDEO_TS/VIDEO_TS.IFO before letting dvdnav to probe, otherwise dvdnav will give error on that and it shows unnecessary error to end user.

This should remove following errors when playing/opening plain directory

[00007f80a4000f20] dvdnav access error: Couldn't find device name.
[00007f80a4000f20] dvdnav access error: DVDOpenFilePath:findDVDFile /VIDEO_TS/VIDEO_TS.IFO failed
[00007f80a4000f20] dvdnav access error: vm: vm: failed to read VIDEO_TS.IFO

It has been long time that I have done string/file-handling in C, so let me know if it looks odd