Segmentation fault with small graph

I tried to create a super small graph to test the implementation but I got a Segmentation fault after the `./vtree_build test.vedge test.vtree`

Graph:
```
2 2
0 1 10
1 0 10
```

Erro:
```
begin read
correct1
2 2
correct2
correct3
correct4
malloc!
Segmentation fault
```