Fix potential panic on AdjacencyMap

[davidovich]

Because the store could theoretically be updated without the use of the main AddVertex and AddEdge APIs (which does some input validation), calculating the AdjencyMap could provoke an "assignment to entry in nil map" panic if the store reported edges that were between unknown vertices.

Since the edges are always between known vertices, it is an error to try and match a missing source vertex from the vertex set. If that happens, it is a data desynchronization error that we report to the user.

In order to test this pathological case, we needed to modify the test to use the store directly because the "front-facing" API would not allow us to add Edges on missing vertex.

fixes #102

Signed-off-by: davidovich [email protected]