Zero palette in case not all entries are initialized later

Originally committed as revision 21384 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vb.c

@@ -254,6 +254,8 @@ static av_cold int decode_init(AVCodecContext *avctx)
     c->frame      = av_malloc( avctx->width * avctx->height);
     c->prev_frame = av_malloc( avctx->width * avctx->height);
 
+    memset(c->pal, 0, sizeof(c->pal));
+
     return 0;
 }