synthesis: Fix segfault when there is no output

* spot/twaalgos/synthesis.cc: here
* tests/python/synthesis.py: create a test
* tests/Makefile.am: add synthesis.py to the tests

tests/Makefile.am

@@ -445,6 +445,7 @@ TESTS_python = \
   python/stutter.py \
   python/rs_like.py \
   python/sum.py \
+  python/synthesis.py \
   python/toparity.py \
   python/trival.py \
   python/tra2tba.py \

tests/python/synthesis.py

@@ -0,0 +1,49 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2021 Laboratoire de Recherche et Développement
+# de l'Epita
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import spot
+
+# A shared variable caused the 2nd call to create_game to give an incorrect
+# result.
+for i in range(0, 2):
+    gi = spot.game_info()
+    gi.s = spot.game_info.solver_LAR
+    game = spot.create_game("(Ga) <-> (Fb)", ["b"], gi)
+    assert not spot.solve_game(game)
+
+# A game can have only inputs
+game = spot.create_game("GFa", [])
+assert(game.to_str() == """HOA: v1
+States: 3
+Start: 0
+AP: 1 "a"
+acc-name: parity max odd 6
+Acceptance: 6 Inf(5) | (Fin(4) & (Inf(3) | (Fin(2) & (Inf(1) | Fin(0)))))
+properties: trans-labels explicit-labels state-acc colored complete
+properties: deterministic
+spot-state-player: 0 1 1
+--BODY--
+State: 0 {1}
+[!0] 1
+[0] 2
+State: 1 {4}
+[t] 0
+State: 2 {5}
+[t] 0
+--END--""")