avcodec/vp6: Use av_freep(), avoid leaving stale pointers

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vp6.c

@@ -651,7 +651,7 @@ static av_cold int vp6_decode_free(AVCodecContext *avctx)
     if (s->alpha_context) {
         ff_vp56_free_context(s->alpha_context);
         vp6_decode_free_context(s->alpha_context);
-        av_free(s->alpha_context);
+        av_freep(&s->alpha_context);
     }
 
     return 0;