smacker: limit recursion depth of smacker_decode_bigtree

This fixes segmentation faults due to stack-overflow caused by too deep
recursion.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/smacker.c

@@ -129,8 +129,12 @@ static int smacker_decode_tree(GetBitContext *gb, HuffContext *hc, uint32_t pref
 /**
  * Decode header tree
  */
-static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc, DBCtx *ctx)
+static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc, DBCtx *ctx, int length)
 {
+    if(length > 500) { // Larger length can cause segmentation faults due to too deep recursion.
+        av_log(NULL, AV_LOG_ERROR, "length too long\n");
+        return AVERROR_INVALIDDATA;
+    }
     if (hc->current + 1 >= hc->length) {
         av_log(NULL, AV_LOG_ERROR, "Tree size exceeded!\n");
         return AVERROR_INVALIDDATA;
@@ -159,12 +163,12 @@ static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc, DBCtx *ctx
         int r = 0, r_new, t;
 
         t = hc->current++;
-        r = smacker_decode_bigtree(gb, hc, ctx);
+        r = smacker_decode_bigtree(gb, hc, ctx, length + 1);
         if(r < 0)
             return r;
         hc->values[t] = SMK_NODE | r;
         r++;
-        r_new = smacker_decode_bigtree(gb, hc, ctx);
+        r_new = smacker_decode_bigtree(gb, hc, ctx, length + 1);
         if (r_new < 0)
             return r_new;
         return r + r_new;
@@ -275,7 +279,7 @@ static int smacker_decode_header_tree(SmackVContext *smk, GetBitContext *gb, int
         goto error;
     }
 
-    if (smacker_decode_bigtree(gb, &huff, &ctx) < 0)
+    if (smacker_decode_bigtree(gb, &huff, &ctx, 0) < 0)
         err = -1;
     skip_bits1(gb);
     if(ctx.last[0] == -1) ctx.last[0] = huff.current++;