closing with no file open results in error

[TLE]

Traceback (most recent call last):
  File "poproofread-gtk.py", line 60, in on_window_destroy
    self.on_mnu_quit(widget)
  File "poproofread-gtk.py", line 102, in on_mnu_quit
    self.ppr.save()
  File "/home/knielsen/code/poproofread/core.py", line 37, in save
    self.file.write(self.content)
AttributeError: PoProofRead instance has no attribute 'file'

Make sure we test if core is active (has a file loaded) before trying
   to save it, e.g. when shutting down the program