Fixes #262, reported by Maximilien Colange.

* bin/common_output.cc, bin/common_aoutput.cc, bin/common_aoutput.hh:
Clear the set of atomic propositions if --stats=%[...]x was used.
* spot/twa/bdddict.cc: Release any formula associated to a BDD when it
is unregistered, do not wait for the dictionary's destruction.  This
was the main culprit for #262.
* tests/core/ltl2tgba.test: Add test cases.
* NEWS: Mention the bug.