Encoder alphanumeric is broken on amd64

`encoder.alphanumeric` doesn't encode the shellcode on amd64.

```
>>> context.binary = ELF('optimistic_patch')
[*] './hackthebox/chall/pwn/Optimistic/optimistic'
Arch: amd64-64-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX disabled
PIE: PIE enabled
RWX: Has RWX segments
>>>
>>> asm(shellcraft.amd64.linux.sh())
b'jhH\xb8/bin///sPH\x89\xe7hri\x01\x01\x814$\x01\x01\x01\x011\xf6Vj\x08^H\x01\xe6VH\x89\xe61\xd2j;X\x0f\x05'
>>> encoder.alphanumeric(asm(shellcraft.sh()))
b'jhH\xb8/bin///sPH\x89\xe7hri\x01\x01\x814$\x01\x01\x01\x011\xf6Vj\x08^H\x01\xe6VH\x89\xe61\xd2j;X\x0f\x05'
```