SIGSEGV caused by wrong `var_order` values

The following code (adapted from the README) produces a SIGSEGV error:

```
from pysdd.sdd import SddManager, Vtree

vtree = Vtree(var_count=2, var_order=[1,1])
a,b = SddManager.from_vtree(vtree).vars

wmc = (a|b).wmc()
```
The cause is apparently the nonsencial `var_order=[1,1]`. Raising a `ValueError` or something would be better than crashing the entire thing. 