-simplify zero-check for age-commitment hash

src/include/taler_crypto_lib.h

@@ -1309,13 +1309,8 @@ struct TALER_AgeAttestation
 #endif
 };
 
-
-extern const struct TALER_AgeCommitmentHash TALER_ZeroAgeCommitmentHash;
-#define TALER_AgeCommitmentHash_isNullOrZero(ph) \
-  ((NULL == ph) ||                               \
-   (0 == memcmp (ph,                             \
-                 &TALER_ZeroAgeCommitmentHash,   \
-                 sizeof(struct TALER_AgeCommitmentHash))))
+#define TALER_AgeCommitmentHash_isNullOrZero(ph) ((NULL == ph) || \
+                                                  GNUNET_is_zero (ph))
 
 /**
  * @brief Type of public signing keys for verifying blindly signed coins.

src/util/crypto.c

@@ -26,11 +26,6 @@
 #include "taler_util.h"
 #include <gcrypt.h>
 
-/**
- * Used in TALER_AgeCommitmentHash_isNullOrZero for comparison
- */
-const struct TALER_AgeCommitmentHash TALER_ZeroAgeCommitmentHash = {0};
-
 /**
  * Function called by libgcrypt on serious errors.
  * Prints an error message and aborts the process.