Fix MainNoGUI abort

I always get this when closing dolphin-emu-nogui:

X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  10 (X_UnmapWindow)
  Resource id in failed request:  0x3400003
  Serial number of failed request:  215
  Current serial number in output stream:  219
terminate called without an active exception
Aborted

I know next to knowing about window management with X but they way I
understand it, the "inner" GLX context thingy is released after its
parent (the window?) has already been closed. No idea if this is the
proper way to fix it.