needs a sanity check in case the icon is missing

[tenabraex]

Traceback (most recent call last):
  File "PDFUnlocker.py", line 116, in <module>
  File "tkinter\__init__.py", line 2155, in wm_iconbitmap
_tkinter.TclError: bitmap "icon.ico" not defined

[ikrahul]

@tenabraex Would be happy to accept a PR if you want to contribute. Should be a very minor change. Also apologies for delayed response, been occupied in work.