pngdec: set reference correctly for corepng.

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

libavcodec/pngdec.c

@@ -506,7 +506,7 @@ static int decode_frame(AVCodecContext *avctx,
                 if(p->data[0])
                     avctx->release_buffer(avctx, p);
 
-                p->reference= 0;
+                p->reference= 1;
                 if(avctx->get_buffer(avctx, p) < 0){
                     av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
                     goto fail;