add num_priv_keys

src/include/taler_crypto_lib.h

@@ -2624,16 +2624,21 @@ struct TALER_AgeCommitment
    * A hash of this list is the hashed commitment that goes into FDC
    * calculation during the withdraw and refresh operations for new coins. That
    * way, the particular age commitment becomes mandatory and bound to a coin.
-   **/
+   */
   struct TALER_AgeCommitmentPublicKeyP *pub;
 
+  /* The number of private keys, which must be at most num_pub_keys.  One minus
+   * this number corresponds to the largest age group that is supported with
+   * this age commitment.
+   */
+  size_t num_priv_keys;
+
   /* A NULL-terminated list of private keys.
    * Note that the list can be _smaller_ than the corresponding list of public
    * keys! In that case, the wallet can sign off only for a subset of the age
    * groups.
    */
   struct TALER_AgeCommitmentPrivateKeyP *priv;
-
 };
 
 /*