Fix: Crash if Movie Clip node has an empty movie

The movie clip GPU texture free function doesn't do null checks, so make
sure the movie clip is not null before freeing.

source/blender/nodes/composite/nodes/node_composite_movieclip.cc

@@ -247,7 +247,9 @@ class MovieClipOperation : public NodeOperation {
   void free_movie_clip_texture()
   {
     MovieClip *movie_clip = get_movie_clip();
-    return BKE_movieclip_free_gputexture(movie_clip);
+    if (movie_clip) {
+      BKE_movieclip_free_gputexture(movie_clip);
+    }
   }
 
   MovieClip *get_movie_clip()