Fix TypeError exception in Python3
Fix TypeError exception in Python3

`va` can be None and this causes Python 3 to raise a TypeError
exception. This is caused by the following breaking change in Python3:
> The ordering comparison operators (<, <=, >=, >) raise a TypeError
> exception when the operands don’t have a meaningful natural ordering.

This didn't failed in the previously tried vivisect version (master from
one week ago and not the release). This may have been caused by a bug in
vivisect that has been fixed.

mandiant/capa,mandiant/capa