Dev9 plugin - prevent null dereference if no device is set

The out of the box settings for pcsx2 is to enable the Ghz Dev9 plugin. This by itself isn't an issue but if you don't manually go into the settings dialog and select a network device then the whole emulator crashes when starting a game (`0` is passed to `InitNet` and then `NetAdapter* nif` is set to 0 which is dereferenced in `NetRxThread`)