Fixes#524, reported by Rüdiger Ehlers.
* spot/twaalgos/dbranch.cc: When merging an edge going to state
without successors simply delete it.
* bin/spot-x.cc: Typo in documentation.
* tests/core/ltlcross.test: Add a test case.
* NEWS: Mention the bug.
Reported by Salomon Sickert.
* spot/twaalgos/complement.cc: Make sure the output of postproc
is deterministic.
* tests/core/ltlcross.test: Add test case.
* NEWS: Mention the bug.