Fix C-j behaviour

In ido-mode C-j is bound to ido-select-text, which will open a new
buffer if a buffer with the given name does not exist.

Previously ffir-ido-find-file would call find-file with nil as the
argument if file was not found in file-list. This would result in an
error.
Merge pull request #5 from martinp/ido-select-text

Fix C-j behaviour