Securely clear buf containing k or its negation

Author: john-moffett

src/modules/schnorrsig/main_impl.h

@@ -191,6 +191,7 @@ static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsi
     secp256k1_scalar_clear(&k);
     secp256k1_scalar_clear(&sk);
     secp256k1_memclear(seckey, sizeof(seckey));
+    secp256k1_memclear(buf, sizeof(buf));
     secp256k1_gej_clear(&rj);
 
     return ret;