Fix #4132 - msfvenom undefined method fullname for NilClass

See https://github.com/rapid7/metasploit-framework/issues/4132

The bug is caused by a bad payload name.

- [x] To verify: without the patch, if you do the following, you will get the undef method error:

```
./msfvenom -p payload/python/meterpreter/reverse_tcp -o LHOST=192.168.130.1 -f raw
```

- [x] With the patch, it should say invalid payload.
- [x] Now, try with a valid payload name: ```./msfvenom -p python/meterpreter/reverse_tcp -o LHOST=192.168.130.1 -f raw```
- [x] It should show you the payload options this time.