smackerdec: Check that the last indexes are within the table.

Fixes CVE-2011-3944

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/smacker.c

@@ -259,6 +259,11 @@ static int smacker_decode_header_tree(SmackVContext *smk, GetBitContext *gb, int
     if(ctx.last[0] == -1) ctx.last[0] = huff.current++;
     if(ctx.last[1] == -1) ctx.last[1] = huff.current++;
     if(ctx.last[2] == -1) ctx.last[2] = huff.current++;
+    if(huff.current > huff.length){
+        ctx.last[0] = ctx.last[1] = ctx.last[2] = 1;
+        av_log(smk->avctx, AV_LOG_ERROR, "bigtree damaged\n");
+        return -1;
+    }
 
     *recodes = huff.values;