For issue #556, 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.