Correctly handle empty frames

Originally committed as revision 12394 to svn://svn.ffmpeg.org/ffmpeg/trunk

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);
     }