Fix some uninitialized pointers in crypto

crypto.c:2748:9: warning: variable 'bn_prime' is used uninitialized whenever '||' condition is true [-Wsometimes-uninitialized]
    if (!get_bn_from_bin(env, argv[0], &bn_verifier)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
crypto.c:2758:6: note: uninitialized use occurs here
        if (bn_prime) BN_free(bn_prime);
            ^~~~~~~~
crypto.c:2748:9: note: remove the '||' if its condition is always false
    if (!get_bn_from_bin(env, argv[0], &bn_verifier)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~