wally_scriptpubkey_get_type: only check 1st byte for OP_RETURN scripts

Even though a script might not be created with wally_scriptpubkey_op_return_from_bytes (e.g. <OP_RETURN>) or it is not standard (e.g. <OP_RETURN OP_RETURN>), if it's starting with OP_RETURN, it should still be considered a OP_RETURN script.

Bitcoin Core checks [1]

    bool IsUnspendable() const
    {
        return (size() > 0 && *begin() == OP_RETURN) || (size() > MAX_SCRIPT_SIZE);
    }

rust-bitcoin does [2]

    pub fn is_op_return(&self) -> bool {
        match self.0.first() {
            Some(b) => *b == OP_RETURN.to_u8(),
            None => false,
        }
    }

[1] https://github.com/bitcoin/bitcoin/blob/d04324a7056a735c1127ba8ccdc720a16e7281a3/src/script/script.h#L552-L555

[2] https://github.com/rust-bitcoin/rust-bitcoin/blob/9df59639cec214bd9363d426335923611a304119/bitcoin/src/blockdata/script/borrowed.rs#L350-L354

ElementsProject / libwally-core

wally_scriptpubkey_get_type: only check 1st byte for OP_RETURN scripts #448