fix: prevent privkey modified by ec_seckey_verify method

[LanfordCai]

ec_seckey_verify method would change the value of the private key inputed as argument, but a verify method should not modify something, moreover, it's a private key. For example:

iex(2)> {pubkey, privkey} = :crypto.generate_key(:ecdh, :secp256k1)
{<<4, 93, 21, 247, 9, 127, 33, 229, 202, 110, 246, 105, 76, 192, 12, 215, 65,
   232, 238, 37, 88, 110, 2, 89, 17, 59, 158, 227, 95, 183, 115, 120, 225, 144,
   170, 19, 9, 196, 68, 64, 96, 90, 78, 173, 112, 46, 89, 88, 71, ...>>,
 <<3, 185, 32, 160, 132, 78, 43, 141, 96, 195, 72, 104, 253, 206, 48, 64, 194,
   120, 210, 131, 182, 147, 112, 208, 189, 114, 155, 211, 62, 248, 22, 158>>}
iex(3)> :ok = :libsecp256k1.ec_seckey_verify(privkey)
:ok
iex(4)> privkey
<<142, 0, 13, 110, 86, 234, 133, 174, 24, 0, 0, 0, 0, 0, 0, 0, 128, 130, 232,
  24, 0, 0, 0, 0, 224, 54, 85, 26, 0, 0, 0, 0>>