Crash on running bfs_verifier

Hello @AjayBrahmakshatriya 

I am trying to run bfs_verifier on included sample graph `4_sym.el`. This gives a crash error.

```bash
$ cd build/bin
$ ./bfs_verifier -f ../../test/graphs/4_sym.el 
# running BFS verifier 
# Segmentation fault (core dumped)
```

I was trying it on web-Stanford graph, both in edge list (.el) and matrix market (.mtx) formats. `graphit_test` passes all tests.