ltlsmin: fix incorrect check for dlsym error

Fix #435 reported by Yann Thierry-Mieg.

* spot/ltsmin/spins_interface.cc (sym): Fix incorrect check introduced
by dc4a477172.

spot/ltsmin/spins_interface.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2019 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE)
+// Copyright (C) 2019, 2020 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE)
 //
 // This file is part of Spot, a model checking library.
 //
@@ -133,7 +133,7 @@ namespace spot
         // should not be converted to pointer-to-functions (we have to
         // assume they can for POSIX).
         *reinterpret_cast<void**>(dst) = lt_dlsym(h, name);
-        if (dst == nullptr)
+        if (*dst == nullptr)
           throw std::runtime_error("Failed to resolve symbol '"s
                                    + name + "' in '" + file + "'.");
       };