Crash when HOME is not set

[Henje]

I am converting a file by running ffmpeg from python via subprocess. This causes all environment variables to be removed.

Running avisynthplus without HOME causes a crash:

terminate` called after throwing an instance of 'std::logic_error'
  what():  basic_string: construction from null is not valid

The crash seems to originate from here:

AviSynthPlus/avs_core/core/avisynth.cpp

Line 2407 in 1d21468

std::string user_avs_plugindir = std::getenv("HOME");