twa: guard against highlighting of non-existing edges and states
For issue #556 (closed), reported by Dávid Smolka. Not sure if this is a complete fix yet, but that's at least part of the issue.
- spot/twa/twagraph.cc (defrag_states): Skip those unexisting edges and states while remaping highlight-states and highlight-edges.
- spot/parseaut/parseaut.yy: Likewise for dropped edges.
- tests/python/parsetgba.py: Augment test case.