Bug in the strongly connected component algorithm

Due to incorrect backtracking on the stack, wrong components are put together using TarjanSCCVisitor.

The initial bug triggering example was the following graph:

nodes: a,b,c,d,e

edges:
a -> b 
b -> c
b -> e
c -> d
d -> b

I expected the following three sccs: {a}, {b,c,d} {e}, but got {a}, {b, e}, {c, d}.