twagraph: fix purge_dead_states on alternating automata

* tests/core/alternating.test: Run some of the test through
valgrind to exhibit the bug.
* spot/twa/twagraph.cc: Fix it.

spot/twa/twagraph.cc

 // -*- coding: utf-8 -*-
-// Copyright (C) 2014, 2015, 2016 Laboratoire de Recherche et Développement de
-// l'Epita.
+// Copyright (C) 2014, 2015, 2016, 2017 Laboratoire de Recherche et
+// Développement de l'Epita.
 //
 // This file is part of Spot, a model checking library.
 //
@@ -300,7 +300,7 @@ namespace spot
           todo.emplace_back(init_number_, 0U, begin, end);
         }
 
-        do
+        for (;;)
           {
             unsigned& tid = std::get<1>(todo.back());
             const unsigned*& begin = std::get<2>(todo.back());
@@ -308,6 +308,8 @@ namespace spot
             if (tid == 0U && begin == end)
               {
                 todo.pop_back();
+                if (todo.empty())
+                  break;
                 unsigned src = std::get<0>(todo.back());
                 if ((int)src >= 0)
                   order.emplace_back(src);
@@ -334,7 +336,6 @@ namespace spot
                 todo.emplace_back(dst, succ, begin, end);
               }
           }
-        while (!todo.empty());
       }
 
     // Process states in topological order

tests/core/alternating.test

 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2016 Laboratoire de Recherche et Développement de
-# l'Epita (LRDE).
+# Copyright (C) 2016, 2017 Laboratoire de Recherche et Développement
+# de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
 #
@@ -319,9 +319,9 @@ State: 3
 --END--
 EOF
 
-autfilt --remove-unreachable-states ex4 > out4
+run 0 autfilt --remove-unreachable-states ex4 > out4
 diff expect4 out4
-autfilt --remove-dead-states ex4 > out4
+run 0 autfilt --remove-dead-states ex4 > out4
 diff ex2 out4
 
 
@@ -338,7 +338,7 @@ State: 1
 --END--
 EOF
 
-autfilt --remove-dead-states ex5 > out5
+run 0 autfilt --remove-dead-states ex5 > out5
 cat >expect <<EOF
 HOA: v1
 States: 1