Reported by Juraj Major.

* spot/twa/twa.hh: check num_sets() in prop_state_acc() so we do not
have to set it in set_acceptance(), causing trouble if set_acceptance()
is called multiple times.
* tests/python/setacc.py: New test case.
* tests/Makefile.am: Add it.
* THANKS: Add Juraj.
* NEWS: Mention the bug.