Crash during undo-redo of edited network

[palvarezlopez]

The crash is caused because after inverting all elements, the connections was selected. After the undo-redo, Netedit try to select the conections agains, but it doesn't' exist and generates an error in NBEdge::getConnectionRef(int fromLane, const NBEdge* to, int toLane).

Bug found in test \selection\bugs\ticket2948 (but not related with #2948)

[palvarezlopez]

Sequence:

1. Create an empty network
2. Create three junctions connected with two edges
3. Compute (this create a connection)
4. Select all elements inverting (this include the connection, and add the selection in undo-stack)
5. Undo all elements
6. Recompute (this destroy the NBEdge connection, but not the GNEEdgeConnection)
7. Redo all. When Netedit try to select the connection again, CRASH, because their associated NBEdge connection doesn't exist

[namdre]

the crash comes faster if you edit an attribute of the connection and then press undo.
It already occurs as far back as 1.2.0