allow model with tags and animation to be exported

[thunderpwn]

Also possible to export an empty model for mapping purposes, sometimes we need an empty model to use with a script mover made out of brushes or other things.

[neumond]

I'm sure len cannot produce values below 0, so this becomes always-false condition, and effectively only suppresses warning. What's the purpose of this change?

[thunderpwn]

well, I am no real coder. But I needed to export md3 with only animation and tags. I am sure you probably know a better way todo it. Also as I said up there, sometimes we need an empty model (no mesh).