KOTOR: Fix usage of dynamic_cast<>()

dynamic_cast<>() to a pointer may return 0, which has
to be checked for.
dynamic_cast<>() to a reference may throw an exception instead.

Coverity issue 1038265.

src/engines/kotor/gui/main/options.cpp

@@ -114,7 +114,7 @@ void OptionsMenu::callbackActive(Widget &widget) {
 }
 
 void OptionsMenu::adoptChanges() {
-	dynamic_cast<OptionsGameplayMenu*>(_gameplay)->adoptChanges();
+	dynamic_cast<OptionsGameplayMenu &>(*_gameplay).adoptChanges();
 }