EventMgr: we need to check that last_win_mousefocused exists

I didnt think it was possible to get a focus lock with no window, but I was wrong.

gemrb/core/GUI/EventMgr.cpp

@@ -180,7 +180,7 @@ void EventMgr::MouseMove(unsigned short x, unsigned short y)
 		// for scrolling
 		gc->OnGlobalMouseMove(x, y);
 	}
-	if (focusLock) {
+	if (last_win_mousefocused && focusLock) {
 		last_win_mousefocused->OnMouseOver(x, y);
 		return;
 	}