Fix CScriptID(const CScript& in) in empty script case

Previously an empty script wouldn't be hashed, and CScriptID would be
assigned the incorrect value of 0 instead. This bug can be seen in the
RPC decodescript command:

    $ btc decodescript ""
    {
        "asm" : "",
        "type" : "nonstandard",
        "p2sh" : "31h1vYVSYuKP6AhS86fbRdMw9XHieotbST"
    }

Correct output:

    $ btc decodescript ""
    {
        "asm" : "",
        "type" : "nonstandard",
        "p2sh" : "3J98t1WpEZ73CNmQviecrnyiWrnqRhWNLy"
    }
Merge pull request #5541

d78f0da Fix CScriptID(const CScript& in) in empty script case (Peter Todd)