Merge bitcoin#960: tests_exhaustive: check the result of secp256k1_ecdsa_sign

a1ee83c tests_exhaustive: check the result of secp256k1_ecdsa_sign (Nicolas Iooss)

Pull request description:

  Hello,

  In `test_exhaustive_sign`, if `secp256k1_ecdsa_sign` fails, the signature which is then loaded by `secp256k1_ecdsa_signature_load` is garbage. Exit early with an error when this occurs.

  By the way, I am wondering whether attribute `SECP256K1_WARN_UNUSED_RESULT` should be added to function `secp256k1_ecdsa_sign`: as (according to the documentation of this function) the nonce generation function may fail, it seems to be a good idea to force callers to check the value returned by this function. What do you think about this?

ACKs for top commit:
  sipa:
    ACK a1ee83c
  real-or-random:
    utACK a1ee83c

Tree-SHA512: d8c186afecbd95522e909c269255e8879695bf9df2de91f0f9303e575e18f03cafc66683d863e6cf9892fe61b668eab00d586861c39013292b71484a962f846d

Author: real-or-random

src/tests_exhaustive.c

@@ -302,6 +302,7 @@ void test_exhaustive_sign(const secp256k1_context *ctx, const secp256k1_ge *grou
             if (skip_section(&iter)) continue;
             for (k = 1; k < EXHAUSTIVE_TEST_ORDER; k++) {  /* nonce */
                 const int starting_k = k;
+                int ret;
                 secp256k1_ecdsa_signature sig;
                 secp256k1_scalar sk, msg, r, s, expected_r;
                 unsigned char sk32[32], msg32[32];
@@ -310,7 +311,8 @@ void test_exhaustive_sign(const secp256k1_context *ctx, const secp256k1_ge *grou
                 secp256k1_scalar_get_b32(sk32, &sk);
                 secp256k1_scalar_get_b32(msg32, &msg);
 
-                secp256k1_ecdsa_sign(ctx, &sig, msg32, sk32, secp256k1_nonce_function_smallint, &k);
+                ret = secp256k1_ecdsa_sign(ctx, &sig, msg32, sk32, secp256k1_nonce_function_smallint, &k);
+                CHECK(ret == 1);
 
                 secp256k1_ecdsa_signature_load(ctx, &r, &s, &sig);
                 /* Note that we compute expected_r *after* signing -- this is important