bppp: Fix test for invalid sign byte

The test is supposed to create an invalid sign byte. Before this PR,
the generated sign byte could in fact be valid due to an overflow.

Co-authored-by: Jonas Nick <jonasd.nick@gmail.com>

Author: real-or-random

src/modules/bppp/tests_impl.h

@@ -256,7 +256,11 @@ static void test_serialize_two_points(void) {
         random_group_element_test(&X);
         random_group_element_test(&R);
         secp256k1_bppp_serialize_points(buf, &X, &R);
-        buf[0] |= 4 + (unsigned char)secp256k1_testrandi64(4, 255);
+
+        buf[0] = 4 + (unsigned char)secp256k1_testrandi64(0, 253);
+        /* Assert that buf[0] is actually invalid. */
+        CHECK(buf[0] != 0x02 && buf[0] != 0x03);
+
         CHECK(!secp256k1_bppp_parse_one_of_points(&X_tmp, buf, 0));
         CHECK(!secp256k1_bppp_parse_one_of_points(&R_tmp, buf, 0));
     }