cdgraphics: do not return 0 from the decode function

0 means no data consumed, so it can trigger an infinite loop in the
caller.

CC:libav-stable@libav.org
(cherry picked from commit c7d9b473)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/cdgraphics.c

@@ -353,10 +353,9 @@ static int cdg_decode_frame(AVCodecContext *avctx,
         *got_frame = 1;
     } else {
         *got_frame = 0;
-        buf_size   = 0;
     }
 
-    return buf_size;
+    return avpkt->size;
 }
 
 static av_cold int cdg_decode_end(AVCodecContext *avctx)