avframe: call release_buffer only if it is set

AVCodecContext release_buffer() shall be NULL for audio codecs using
get_buffer. The backward compatibility code hence have to check before
calling it.

libavcodec/utils.c

@@ -535,7 +535,8 @@ typedef struct CompatReleaseBufPriv {
 static void compat_free_buffer(void *opaque, uint8_t *data)
 {
     CompatReleaseBufPriv *priv = opaque;
-    priv->avctx.release_buffer(&priv->avctx, &priv->frame);
+    if (priv->avctx.release_buffer)
+        priv->avctx.release_buffer(&priv->avctx, &priv->frame);
     av_freep(&priv);
 }