Fix panic if gremlin query At() and As() were used
Fix panic if gremlin query At() and As() were used

Query "G.At(0).V().As('x')" was crashing the analyzer because
a "assignment to entry in nil map".

GraphTraversal.as was not initializated after cloning the graph in the
"At/Context" step.
"As" step produced the panic while trying to assign a value to the
uninitializated map

skydive-project/skydive,skydive-project/skydive,skydive-project/skydive,skydive-project/skydive,skydive-project/skydive,skydive-project/skydive,skydive-project/skydive