work around some g++-7.3 issues

* spot/twaalgos/langmap.cc, spot/ltsmin/ltsmin.cc: Add asserts to hide
spurious "potential null pointer dereference" messages.
* spot/twaalgos/tau03opt.cc (color_ref): Initialize pc to nullptr
even when is_cyan is true so that valgrind does not report pc being
used uninitialized in is_white().

spot/twaalgos/langmap.cc

@@ -44,6 +44,7 @@ namespace spot
     for (unsigned i = 0; i < n_states; ++i)
       {
         twa_graph_ptr c = make_twa_graph(aut, twa::prop_set::all());
+        assert(c); // for some reason g++ 7.3 thinks this could be null
         c->set_init_state(i);
         alt_init_st_auts[i] = c;
         compl_alt_init_st_auts[i] = remove_fin(dualize(c));