MIPS exec payload fixes for encoder

A little fix for our MIPS exec shellcodes to fulfill the 4 byte alignment. Till now we had the following exception as soon as we try to use an encoder:

    #./msfpayload linux/mipsbe/exec CMD=/bin/bash R | ./msfencode -e mipsbe/longxor -t c
    [-] mipsbe/longxor failed: uninitialized constant       Msf::Modules::Mod656e636f6465722f6d69707362652f6c6f6e67786f72::Metasploit3::InvalidPayloadSizeException
    [-] No encoders succeeded.

Now something like this will work ;)

Best,
Mike