Another crash fix for duplicate edges. Fixes #939.

Another crash fix for duplicate edges. Fixes #939.

Patch #933 fixed a crash with duplicate edges by not adding edges to the
graph if all the edge's outputs are already built by other edges.
However, it added the edge to the out_edges of the edge's input nodes
before deleting it, letting inputs refer to dead edges.

To fix, move the check for deleting an edge above the code that adds
inputs.  Expand VerifyGraph() to check that nodes don't refer to edges
that aren't present in the state.