Fix badchars usage in msfvenom

msfvenom was interpreting the badchar string "\x20\x00\x12" as containing '\' and 'x' literals. Not sure when this was introduced, but here is a fix.

##### Verification

- [x] Generate the MIPS bind payload for linux with some badchars using msfvenom.

        ./msfvenom -p linux/mipsle/shell_bind_tcp -f elf -a mipsle -b'\x20\x00\x12' --platform linux