twa: fix intersecting_run on weak automata

Fixes #471, reported by Cambridge Yang.

* spot/twa/twa.cc (intersecting_run): Disable the product
optimization for weak automata.
* tests/python/471.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the bug.

tests/Makefile.am

@@ -380,6 +380,7 @@ TESTS_ipython = \
 TESTS_python = \
   python/298.py \
   python/341.py \
+  python/471.py \
   python/_altscc.ipynb \
   python/_autparserr.ipynb \
   python/_aux.ipynb \

tests/python/471.py

@@ -0,0 +1,28 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2021 Laboratoire de Recherche et Développement de l'Epita
+# (LRDE).
+#
+# 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/>.
+
+# Test for Issue #471.
+
+import spot
+a = spot.translate('Fa')
+a = spot.to_generalized_rabin(a, False)
+r1 = a.intersecting_run(a)
+r2 = a.accepting_run()
+assert str(r1) == str(r2)
+assert a.prop_weak().is_true()