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" }