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"
    }

Rebased-From: d78f0dafd5
Github-Pull: #5541

src/script/standard.cpp

@@ -18,7 +18,7 @@ typedef vector<unsigned char> valtype;
 
 unsigned nMaxDatacarrierBytes = MAX_OP_RETURN_RELAY;
 
-CScriptID::CScriptID(const CScript& in) : uint160(in.size() ? Hash160(in.begin(), in.end()) : 0) {}
+CScriptID::CScriptID(const CScript& in) : uint160(Hash160(in.begin(), in.end())) {}
 
 const char* GetTxnOutputType(txnouttype t)
 {