Ignore video track `vmhd` as it may be cover image

Why do you even care that you found video track?
I'm listening to `Above & Beyound: Group Therapy` podcast, and for example this correct AAC file: http://traffic.libsyn.com/anjunabeats/ABGT086_76345674.m4a
have `vmhd` inside, but it is only JPEG cover, and it is OK.
Moreover, `vmhd` is placed before other meta information, and having such check you get no metadata at all.