Correctly handle empty frames

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@12394 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/zmbv.c

@@ -504,6 +504,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, const
     } else {
         c->pic.key_frame = 0;
         c->pic.pict_type = FF_P_TYPE;
+        if(c->decomp_len)
         c->decode_xor(c);
     }