Graph dominiator tree considers unvisited vertices

[scott.gregory.west@…]

The graph dominator tree will consider vertices that are not reachable from the given root. The unreachable check is implemented checking "dfnum < 0

dfnum >= numOfVertices", but the dfnum map is initialized to 0. So when nodes are really unvisited they will have a value of 0 and pass the check.

Simple fix could be: ​https://github.com/boostorg/graph/pull/97