Fix bug #24477: Segfault when launching credits screen

Guifixes introduced a regression where the code would try to iterate
over a pointer to a vector without checking if it was non-null. This
adds the check to ensure that it is non-null before attempting to
iterate.

src/show_dialog.cpp

@@ -169,8 +169,10 @@ void dialog_frame::handle_event(const SDL_Event& event) {
 	if (event.type == DRAW_ALL_EVENT) {
 		set_dirty();
 
-		for(std::vector<button *>::iterator it = buttons_->begin(); it != buttons_->end(); ++it) {
-			(*it)->set_dirty(true);
+		if (buttons_) {
+			for(std::vector<button *>::iterator it = buttons_->begin(); it != buttons_->end(); ++it) {
+				(*it)->set_dirty(true);
+			}
 		}
 	}