Clear a copied secret key after negation

src/secp256k1.c

@@ -567,6 +567,7 @@ int secp256k1_ec_privkey_negate(const secp256k1_context* ctx, unsigned char *sec
     secp256k1_scalar_negate(&sec, &sec);
     secp256k1_scalar_get_b32(seckey, &sec);
 
+    secp256k1_scalar_clear(&sec);
     return 1;
 }