Prevent a crash when calling AnimationMixer::restore when an invalid reference

(cherry picked from commit 937411e152f5e84c0f4a59d59e2a813ecda8dd06)

scene/animation/animation_mixer.cpp

@@ -1985,6 +1985,7 @@ void AnimationMixer::reset() {
 }
 
 void AnimationMixer::restore(const Ref<AnimatedValuesBackup> &p_backup) {
+	ERR_FAIL_COND(p_backup.is_null());
 	track_cache = p_backup->get_data();
 	_blend_apply();
 	track_cache = HashMap<NodePath, AnimationMixer::TrackCache *>();