ltlcross: fix --verbose --no-check crash

Report from František Blahoudek.

* bin/ltlcross.cc: Do not display stats for automata
that do not exist.
* tests/core/ltlcross3.test: Test it.
* NEWS: Mention the fix.

NEWS

@@ -32,6 +32,8 @@ New in spot 2.1.1.dev (not yet released)
   * Fix python errors on Darwin when using methods from the spot module
     inside of the spot.ltsmin submodule.
 
+  * Fix ltlcross crash when combining --no-check with --verbose.
+
 New in spot 2.1.1 (2016-09-20)
 
   Command-line tools:

bin/ltlcross.cc

@@ -1066,6 +1066,8 @@ namespace
           auto tmp = [&](std::vector<spot::twa_graph_ptr>& x, unsigned i,
                          const char prefix)
             {
+              if (!x[i])
+                return;
               std::cerr << "info:   " << prefix << i << "\t(";
               printsize(x[i]);
               std::cerr << ')';
@@ -1094,7 +1096,7 @@ namespace
                              std::vector<spot::twa_graph_ptr>& to, unsigned i,
                              char prefix)
                 {
-                  if (!to[i])
+                  if (from[i] && !to[i])
                     {
                       if (print_first)
                         {

tests/core/ltlcross3.test

 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2012, 2013, 2014, 2015 Laboratoire de Recherche et
-# Développement de l'Epita (LRDE).
+# Copyright (C) 2012, 2013, 2014, 2015, 2016 Laboratoire de Recherche
+# et Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
 #
@@ -180,3 +180,7 @@ cat >expected<<EOF
 %
 EOF
 diff foo expected
+
+
+# This command used to crash.  Report from František Blahoudek.
+run 0 ltlcross --verbose --no-checks -f 'FGa' 'ltl2tgba'