* spot/twaalgos/minimize.cc (minimize_wdba): Diminish the color of
terminal SCCs that are incomplete, as if they had a non-accepting
sink as successor.
* spot/twaalgos/strength.hh, spot/twaalgos/strength.cc
(is_terminal_automaton): Add an option to ignore trivial SCC as we did
before, since it matters for deciding membership to the guarantee
class.
(is_safety_mwdba): Rewrite as ...
(is_safety_automaton): ... generalizating to any acceptance, and
ignoring trivial SCCs.
* bin/ltlfilt.cc, python/ajax/spotcgi.in, spot/tl/hierarchy.cc,
tests/core/ikwiad.cc: Adjust usage of is_terminal_automaton and
is_safety_automaton().
* tests/core/hierarchy.test: Add a problematic formula as test-case.
* NEWS: Mention the bug.