test: improve error handling in x509_set_serial_check()

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

tests/suites/test_suite_x509write.function

@@ -439,6 +439,8 @@ void x509_set_serial_check()
                                        sizeof(invalid_serial)), 0);
     TEST_EQUAL(mbedtls_x509write_crt_set_serial(&ctx, &serial_mpi),
                MBEDTLS_ERR_X509_BAD_INPUT_DATA);
+
+exit:
     mbedtls_mpi_free(&serial_mpi);
 }
 /* END_CASE */