Merge #1650: schnorrsig: clear out masked secret key in BIP-340 nonce…

… function

a82287f schnorrsig: clear out masked secret key in BIP-340 nonce function (Sebastian Falbesoner)

Pull request description:

ACKs for top commit:
  real-or-random:
    utACK a82287f
  jonasnick:
    ACK a82287f

Tree-SHA512: 0e77ddc299e204edae238759e549d4e8314abb730a654580a109ec05daf53a625be1cc37664a9e00fd41cf34a94abede96b547e661cdf18c40c50141e7b4ee0e

src/modules/schnorrsig/main_impl.h

@@ -94,6 +94,8 @@ static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *ms
     secp256k1_sha256_write(&sha, msg, msglen);
     secp256k1_sha256_finalize(&sha, nonce32);
     secp256k1_sha256_clear(&sha);
+    secp256k1_memclear(masked_key, sizeof(masked_key));
+
     return 1;
 }