qprintdialog_unix.cpp: fix out-of-bounds when cwd is empty.

This happens when the cwd got deleted meanwhile; this warning is printed out:
QFileSystemEngine::currentPath: getcwd() failed
and currentPath() is empty. Not a problem in itself, but qprintdialog
should not assume it's not empty, then.

Change-Id: I5c89992b36ca58b6325ced399cff33aca9efb7ec
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>