Turn on some new warnings from GCC 6.0

* m4/gccwarn.m4: Add the new warnings.
* spot/ta/taexplicit.cc, spot/ta/taproduct.cc: Add missing asserts.

m4/gccwarn.m4

@@ -35,7 +35,11 @@ EOF
    Wdocumentation \
    Wmissing-declarations \
    Wnoexcept \
-   Woverloaded-virtual
+   Woverloaded-virtual \
+   Wmisleading-indentation \
+   Wlogical-op \
+   Wduplicated-cond \
+   Wnull-dereference
   do
     CXXFLAGS="$cf_save_CXXFLAGS $ac_cv_prog_gxx_warn_flags -$cf_opt"
     if AC_TRY_EVAL(ac_compile); then

spot/ta/taexplicit.cc

 // -*- coding: utf-8 -*-
-// Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire de
-// Recherche et Développement de l'Epita (LRDE).
+// Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015, 2016 Laboratoire
+// de Recherche et Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -362,7 +362,7 @@ namespace spot
       {
         auto* s = const_cast<state_ta_explicit*>
 	  (down_cast<const state_ta_explicit*>(*it));
-
+	assert(s);
         s->free_transitions();
         s->get_tgba_state()->destroy();
         delete s;
@@ -383,6 +383,7 @@ namespace spot
   ta_explicit::add_to_initial_states_set(state* state, bdd condition)
   {
     state_ta_explicit* s = down_cast<state_ta_explicit*>(state);
+    assert(s);
     s->set_initial_state(true);
     if (condition == bddfalse)
       condition = get_state_condition(s);

spot/ta/taproduct.cc

 // -*- coding: utf-8 -*-
-// Copyright (C) 2011, 2012, 2014, 2015 Laboratoire de Recherche et
-// Développement de l'Epita (LRDE).
+// Copyright (C) 2011, 2012, 2014, 2015, 2016 Laboratoire de Recherche
+// et Développement de l'Epita (LRDE).
 //
 //
 // This file is part of Spot, a model checking library.
@@ -373,6 +373,7 @@ namespace spot
   ta_product::is_hole_state_in_ta_component(const spot::state* s) const
   {
     const state_ta_product* stp = down_cast<const state_ta_product*> (s);
+    assert(stp);
     ta_succ_iterator* ta_succ_iter = get_ta()->succ_iter(stp->get_ta_state());
     bool is_hole_state = ta_succ_iter->done();
     delete ta_succ_iter;