#47 fatglue.py fails with a single executable

[Duane Murphy]

Running fatglue.py as:

fatglue.py dest.efi source.efi

results in an error from os.path.exists() that NoneType cannot be cooerced to a Unicode string.

Fix:
Change line 17 from
if not os.path.exists(srcfile2):
to
if srcfile2 and not os.path.exists(srcfile2):