Infinite loop caused by nested sub-graphs

[badams]

Creating a graph with nesting of more than two levels causes an infinite loop.

Steps to reproduce

Create nested graph structure like so.

$graph = new Digraph('G');
$graph->node('A')->edge(['A', 'B']);

$sub1 = $graph
    ->subgraph('cluster_1')
    ->node('B')
    ->edge(['B', 'C']);

$sub2 = $sub1->subgraph('cluster_2')
    ->node('C')
    ->edge(['C', 'D']);

$sub3 = $sub2->subgraph('cluster_3')
    ->node('D');

The final call to Subgraph::createEdge() will create an infinite loop as it is unable to resolve the top level graph.

[alexandresalome]

Fixed in version 2