Fix null-pointer dereference

I had never seen `face_tracker_dlib::set_position` was called without
it's texture for many years. However, I saw `p->tex` was NULL while
developing another code. It would be safe to ensure the pointer is
valid.

src/face-tracker-dlib.cpp

@@ -60,6 +60,8 @@ void face_tracker_dlib::set_texture(std::shared_ptr<texture_object> &tex)
 
 void face_tracker_dlib::set_position(const rect_s &rect)
 {
+	if (!p->tex)
+		return;
 	p->rect.x0 = rect.x0 / p->tex->scale;
 	p->rect.y0 = rect.y0 / p->tex->scale;
 	p->rect.x1 = rect.x1 / p->tex->scale;