avcodec/rl2: clear freed pointers

Avoid leaving stale pointers in memory
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/rl2.c

@@ -207,7 +207,7 @@ static av_cold int rl2_decode_end(AVCodecContext *avctx)
 {
     Rl2Context *s = avctx->priv_data;
 
-    av_free(s->back_frame);
+    av_freep(&s->back_frame);
 
     return 0;
 }