netfilter: nft_set_bitmap: initialize set element extension in lookups

Otherwise, nft_lookup might dereference an uninitialized pointer to the
element extension.

Fixes: 665153ff ("netfilter: nf_tables: add bitmap set type")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_set_bitmap.c

@@ -81,6 +81,7 @@ static bool nft_bitmap_lookup(const struct net *net, const struct nft_set *set,
 	u32 idx, off;
 
 	nft_bitmap_location(set, key, &idx, &off);
+	*ext = NULL;
 
 	return nft_bitmap_active(priv->bitmap, idx, off, genmask);
 }