fixup! stack overflow in generators_serialize test

otherwise memset inside generators_serialize writes outside the bounds of the
gens_ser array

Author: jonasnick

src/modules/bulletproofs/tests_impl.h

@@ -44,12 +44,18 @@ static void test_bulletproofs_generators_api(void) {
     CHECK(!secp256k1_bulletproofs_generators_serialize(none, gens, gens_ser, &len));
     len = sizeof(gens_ser) - 1;
     CHECK(!secp256k1_bulletproofs_generators_serialize(none, gens, gens_ser, &len));
-    len = sizeof(gens_ser) + 1; /* len can be greater than minimum needed */
-    CHECK(secp256k1_bulletproofs_generators_serialize(none, gens, gens_ser, &len));
-    CHECK(len == sizeof(gens_ser));
-    CHECK(ecount == 3);
+    len = sizeof(gens_ser);
+    {
+        /* Output buffer can be greater than minimum needed */
+        unsigned char gens_ser_tmp[331];
+        size_t len_tmp = sizeof(gens_ser_tmp);
+        CHECK(secp256k1_bulletproofs_generators_serialize(none, gens, gens_ser_tmp, &len_tmp));
+        CHECK(len_tmp == sizeof(gens_ser_tmp) - 1);
+        CHECK(ecount == 3);
+    }
 
     /* Parse */
+    CHECK(secp256k1_bulletproofs_generators_serialize(none, gens, gens_ser, &len));
     ecount = 0;
     gens = secp256k1_bulletproofs_generators_parse(none, NULL, sizeof(gens_ser));
     CHECK(gens == NULL && ecount == 1);