flacdec: don't call allocate_buffers() in init_thread_copy() when s->max_blocksize is 0

This is normal, and happens when no extradata was available at decoder
initialization.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/flacdec.c

@@ -574,7 +574,9 @@ static int init_thread_copy(AVCodecContext *avctx)
     FLACContext *s = avctx->priv_data;
     s->decoded_buffer = NULL;
     s->decoded_buffer_size = 0;
-    return allocate_buffers(s);
+    if (s->max_blocksize)
+        return allocate_buffers(s);
+    return 0;
 }
 
 static av_cold int flac_decode_close(AVCodecContext *avctx)