Assertion "logger != nullptr failed on shutdown

[GunChleoc]

This happens when Widelands takes too long to shut down the sound system, because the system is busy:

SoundHandler closing times 1, freq 22050, format 32784, chan 2
SDL_AUDIODRIVER pulseaudio
widelands: /home/bratzbert/sources/widelands/trunk/src/base/log.cc:151: void log(const char*, ...): Assertion `logger != nullptr' failed.
Aborted (core dumped)

[GunChleoc (gunchleoc)]

Fixed in build20-rc1