add assertions after many unchecked dynamic casts

These were all reported by coverity, and indeed seem to have been
assumed to work without failing at runtime.

src/game_initialization/create_engine.cpp

@@ -811,6 +811,8 @@ void create_engine::set_current_level(const size_t index)
 		random_map* current_random_map =
 			dynamic_cast<random_map*>(&current_level());
 
+		assert(current_random_map); // if dynamic cast has failed then we somehow have gotten all the pointers mixed together.
+
 		generator_.reset(current_random_map->create_map_generator());
 	} else {
 		generator_.reset(NULL);

src/game_initialization/multiplayer_create.cpp

@@ -368,6 +368,8 @@ void create::process_event()
 			ng::scenario* current_scenario =
 				dynamic_cast<ng::scenario*>(&engine_.current_level());
 
+			assert(current_scenario);
+
 			players << current_scenario->num_players();
 			map_size << _("Size: ") << current_scenario->map_size();
 
@@ -378,6 +380,8 @@ void create::process_event()
 			 ng::campaign* current_campaign =
 				dynamic_cast<ng::campaign*>(&engine_.current_level());
 
+			assert(current_campaign);
+
 			players << current_campaign->min_players();
 			if (current_campaign->max_players() !=
 				current_campaign->min_players()) {

src/gui/widgets/widget.cpp

@@ -96,12 +96,12 @@ twidget::~twidget()
 
 void twidget::set_id(const std::string& id)
 {
+	tcontrol* this_ctrl = dynamic_cast<tcontrol*>(this);
+
 	DBG_GUI_LF
 	<< "set id of " << static_cast<void*>(this) << " to '" << id << "' "
 	<< "(was '" << id_ << "'). Widget type: "
-	<< (dynamic_cast<tcontrol*>(this)
-				? dynamic_cast<tcontrol*>(this)->get_control_type()
-				: typeid(twidget).name()) << "\n";
+	<< (this_ctrl ? this_ctrl->get_control_type() : typeid(twidget).name()) << "\n";
 
 	id_ = id;
 }

src/scripting/game_lua_kernel.cpp

@@ -2781,6 +2781,9 @@ static int intf_debug_ai(lua_State *L)
 		//so set up a dummy engine
 
 		ai::ai_composite * ai_ptr = dynamic_cast<ai::ai_composite *>(c);
+
+		assert(ai_ptr);
+
 		ai::ai_context& ai_context = ai_ptr->get_ai_context();
 		config cfg = ai::configuration::get_default_ai_parameters();