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>

src/modules/bppp/tests_impl.h

@@ -254,10 +254,15 @@ void test_serialize_two_points(void) {
     {
         secp256k1_ge X_tmp, R_tmp;
         unsigned char buf[65];
+        unsigned char invalid_sign;
         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));
     }