twa: do not set prop_state_acc in set_acceptance
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.