Merge bitcoin#933: Avoids a missing brace warning in schnorrsig/tests_impl.h on old compilers

99e2d5b Avoids a missing brace warning in schnorrsig/tests_impl.h on old compilers. (Gregory Maxwell)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 99e2d5b
  jonasnick:
    utACK 99e2d5b

Tree-SHA512: f3f9cfcd62830d7accca74dfce40abb091dec0990a66bad5d2a9599f2533121d8d1422499d511512bfb8d7c57da96e29e012dbc210e2e97ad55ad18de0869735

Author: jonasnick

src/modules/schnorrsig/tests_impl.h

@@ -103,7 +103,7 @@ void test_schnorrsig_api(void) {
     unsigned char sk3[32];
     unsigned char msg[32];
     secp256k1_keypair keypairs[3];
-    secp256k1_keypair invalid_keypair = { 0 };
+    secp256k1_keypair invalid_keypair = {{ 0 }};
     secp256k1_xonly_pubkey pk[3];
     secp256k1_xonly_pubkey zero_pk;
     unsigned char sig[64];